MAJOR code overhaul: ambient noise adjoint tomography and code improvements

[bch0w]

WARNING: Many backwards incompatible changes will be introduced to SeisFlows, likely breaking the devel branch. In the coming months I aim to clean up, test and document these changes so they can be incorporated into SeisFlows version 3.0.0.

The gist:

• This is a big PR that attempts to incorporate an ongoing code overhaul and feature implementation into the main devel branch of SeisFlows.
• Started as a new workflow for inverting for empirical Green's functions (ambient noise cross correlations) using SeisFlows following the methodologies of Wang et al..
• Eventually became apparent that many of the existing structures within SeisFlows needed to be modified to accommodate this.
• Additional non-relevant features and bug fixes were added on top of the new feature

Before I let this feature branch get so unruly that it cannot be easily merged back into devel, I am opting to PR it now, and to continue improving the devel branch through smaller feature PRs.

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The changelog so far includes the following (also found in CHANGELOG.md). I've tried to highlight (boldface) the changes I think might be most interesting/useful/critical:

Major Changes

• Workflow:
  • State file changed states from words to integers (completed -> 1,
    failed -> -1, pending -> 0), and full state file is created at workflow
    setup, rather than one by one as each function completes
  • Inversion:
    • Changed total misfit summation from summed + squared to L1 norm.
    • Line search has been broken into multiple functions to facilitate
      restarting failed line searches:
      perform_line_search() -> evaluate_line_search_misfit() + update_line_search()
  • NoiseInversion:
    • Modifies the Inversion class to invert for ZZ, RR and TT empirical
      Green's functions
    • Utility function to rotate EE, EN, NE, NN waveforms to RR and TT
    • Utility function to rotate RR and TT adjoint sources to EE, EN, NE, NN
    • Utility function to convert STATIONS file to N SOURCE files since
      virtual sources are required for ambient noise adjoint tomography
    • Functionality to create a source-receiver lookup table that contains
      information on azimuth + backazimuth required for RR/TT kernels
• Preprocessing:
  • Default (major upgrades)
    • Force 'obs' data to match 'syn' data sampling rate
    • Parallelized misfit quantification with concurrent futures
    • Zero'd adjoint source generation now occurs at module setup
      (parallelized)
    • Added additional normalization options
    • Allows selection by component for misfit quantification
    • Improved obs-syn file match validation
    • Removed the ability to sum_residuals, which required Preprocess to
      know too many things about the workflow. Now handled by Workflow.
    • Added a simple waveform plotter to show obs, syn and adjoint source
    • Split off preprocessing functions (mute, normalize) to tools which
      Preprocess can import
  • Pyaflowa
    • Removed client parameter to match Pyatoa > 0.3.0
    • Allow processing only for specific components
    • Data reading abstraction simplified, no longer builds paths from parts
      but instead explcitely reads data + metadata like Default preproc.
    • Pyflex preset now directly part of parameter file so that User can
      edit them directly
• System:
  • General:
    • tasktime now set in the top parent class
    • Allow custom tasktimes for functions run with System.run
    • rerun function tells System to re-run failed jobs some number of
      times to deal with randomly failing tasks that usually work once you
      run them again; added to TestFlow
  • Cluster (and derived classes):
    • New parameter array: For debug purposes, allow running only specific
      task IDs to e.g., re-run failed processes. Input style follows SLURM
      array argument
    • Submit jobs directly to the login node with the -l/--login flag
    • Non-zero exit code error catching added to concurrent future calls
    • Overhauled job monitoring system. Notably, does not break on first
      job failure, but rather to wait until all jobs are finished. Tied into System rerun feature
  • Slurm (and derived classes):
    • Added a timeout counter and extended timeout value for checking
      output of sacct for queue checking due to premature job exits with
      empty sacct returns (i.e., it takes a while for compute nodes to
      spin up and be visible in sacct)
• Optimize:
  • Major reorganization, breaking major monolithic functions into smaller pieces
  • Improved function and checkpoint order with the intent of easier failure recovery
  • Internal restart and revert functions for manually stepping back failed line searches
• Solver:
  • API change: solver.combine() made more generic and no longer hardcodes
    assumed directory structure
  • Parameter change: density -> update_density
  • Model parameter checks removed from Solver's abilities. These are now
    handled by Workflow
  • Takes over responsibility for renaming adjoint sources
  • Takes over responsibility for obs-syn filename matching prior to preproc.
• Command Line Tool:
  • seisflows submit --login -> seisflows submit --direct for submitting
    your workflow directly to the login/home node and not to a cluster node
  • seisflows configure makes more clear which paths are default/not
    important, and which paths are required
  • seisflows setup <-> seisflows init namespace change as the names make
    more sense in this order. init starts a blank working directory, setup
    runs module setup functions (like directory creation)
• New Dependencies:
  • PyPDF: for PDF mergers in Pyaflowa preprocessing
  • PySEP: for SPECFEM-specific read functions

Minor Changes

• Solver:
  • Parallelized directory initialization w/ concurrent futures
  • Kernel renaming defined as a separate function (previously part of
    adjoint simulation), so that it can be called by debugger
• Optimization:
  • Improves step length overwrite log messaging
  • Skips initial line search calculation if initial step length requested
• Workflow: Allow other workflows to overwrite the default location where
  synthetic waveforms are saved
• System: improved file system management by organizing spawned process log files
  and removing scratch/system directory each iteration
• Preprocessing Pyaflowa: improved output file management, export and removal
• Model checking now occurs in Workflow rather than Solver functions
• Workflow data preparation now symlinks in real data rather than copies it,
  to avoid heavy file overhead
• Removed unnused graphics utilities and adopted all of Pyatoa's PNG and PDF
  image manipulation utilities for use in Pyaflowa preprocessing (adds PyPDF as
  dependency)
• Removed large sections of commented out code from command line tool
• Single source version number in pyproject.toml
• Logger aesthetic change to show first four letters of message type rather than
  first letter (e.g., I -> INFO, W -> WARN, D -> DEBU)
• Model parameter check now includes mean values in addition to min and max
• New CLI tool: seisflows print tasks to get source names and relevant task ID
• Model tool now breaks on check if any NaNs present in model arrays
• Optimize: split up some internal functions for easier separation of tasks
  that were previously all mashed together

Bugfixes

• Major: SPECFEM3D_GLOBE based solvers were NOT updating the model during
  Inversion workflows ecause xmeshfem3D was not being called, and therefore
  not updating database files.
• Pyaflowa was checking the incorrect values for windows returned from Manager
  causing incorrect misfit calculation and an inability for the inversion to reduce misfit
• Cross-correlation Traveltime misfit function was not squared, allowing CC
  values to be negative. Now follows Tromp (2005) where we square the time shift
• mpiexec was being set inside System initiation, causing check statement to
  fail quietly
• System.Cluster.Run now passes User-defined arguments for log level and
  verbosity to each child process allowing for uniform logs for all jobs
• seisflows swap allow paths to be set relative or absolute, previously they
  were forced to absolute
• Old Optimization files (e.g., m_old) were not being deleted due to missing
  file extensions. Not critical because they were not used, and overwritten
• seisflows setpar was not properly setting FORTRAN double precision values.
  Added some better catches for setpar as it was quietly failing when files
  were nonexistent
• LBFGS line search restart step_count_max was not being evaluated properly
• seisflows configure will no longer try to configure an already configured
  file
• Concurrency: better all-around error catching for any functions that are
  parallelized by concurrent futures. Previously these functions failed quietly
• solver.specfem3d_globe was not recognizing custom model types

Misc.

• Removed hard requirement that import_seisflows required all Workflows have
  modules as their first argument. Only Forward workflow requires.
• Removed Optimize load checkpoint from inversion setup because it was already
  run by Optimize setup