Usability improvements for Python-based workflows

[tcmetzger]

The Python interface also makes pyRTE+RRTMGP easier to use in Python-based workflows. Specifically:

• Establish conventions: define a set of conventions for gas names, temperatures, pressures, and potentially other physical quantities, based on FC standards Version 27 (http://cfconventions.org/Data/cf-standard-names/27/build/cf-standard-name-table.html), see Python front-end: Conventions for user facing code #47 for more details!
• Inferring towards those conventions: Use xarray metadata to extract information about the data available in the xarray data set and make it accessible in a standardized way on the frontend (based on the established conventions)
• Wrapping:
  • Update the Python interface to use verbs instead of nouns (solve_lw instead of lw_solver).
  • Add a rte.solve function that automatically infers which solver to use based on the user-provided input data and uses sensible defaults. Specifically, this automates the following two decisions:
    • Retain spectral detail or accumulate on the fly?
    • Is the problem a 1-parameter or 3-paramter problem?
  • Robert and team to provide documentation about how the Fortran interface handles inferring from user input and mapping these choices to the kernel arguments

Related to #47