Add tach package to dev for dependency analysis

[deltamarnix]

Creates visualizations of the import statements in the code. See DEVELOPER.md on how to use it.

[deltamarnix]

I think this is a very handy tool to generate mermaid files with information on the dependencies between modules internal to flopy. I have committed the tach.toml as a starting point with settings that help to analyze the mf6 module and the utility packages surrounding it.

If you think this has value, merge it. If this is too auxiliary for what flopy stands for, I understand.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.5%. Comparing base (bb9824e) to head (548f479).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2373     +/-   ##
=========================================
+ Coverage     68.4%   74.5%   +6.1%     
=========================================
  Files          294     292      -2     
  Lines        59390   59623    +233     
=========================================
+ Hits         40652   44468   +3816     
+ Misses       18738   15155   -3583     

see 227 files with indirect coverage changes

[wpbonelli]

seems useful and maybe encourages explicit decisions about visibility?