refactor(modpathfile): step toward generic particle tracking api

[wpbonelli]

Refactoring in preparation to support PRT output files under a unified interface.

• introduce particletracking module and ParticleTrackFile base class
• rename _ModpathSeries -> ModpathFile
• deduplicate shared logic in ModpathFile
• add dtypes as public class attributes
• clarify canonical (minimal) fields
• misc cleanup in plotutil.py

The PR avoids breaking changes except one: since all ParticleTrackFile children will share the base implementation of write_shapefile() the first param becomes simply data. While this may break uses of pathline_data/etc named params, it seems like a small enough change to make in the next minor release edit: see comment below

[codecov]

Codecov Report

Attention: Patch coverage is 94.82072% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 73.1%. Comparing base (b142b08) to head (a138a72).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2127     +/-   ##
=========================================
+ Coverage     71.3%   73.1%   +1.7%     
=========================================
  Files          259     260      +1     
  Lines        59511   59411    -100     
=========================================
+ Hits         42479   43444    +965     
+ Misses       17032   15967   -1065     

Files	Coverage Δ
flopy/plot/plotutil.py	56.0% <100.0%> (+<0.1%)	⬆️
flopy/utils/flopy_io.py	75.3% <ø> (-0.1%)	⬇️
flopy/utils/particletrackfile.py	96.0% <96.0%> (ø)
flopy/utils/modpathfile.py	88.4% <93.8%> (+1.4%)	⬆️

... and 61 files with indirect coverage changes

[wpbonelli]

on second thought I restored the write_shapefile functions/params and added a deprecation warning instead — probably best to follow semver and wait for the next major version to introduce a breaking change