ci: fix optional dependency workflow

[wpbonelli]

• add some explanatory comments to optional.yml
• check for shapely and scipy before creating voronoi test grids (fix failing optional.yml workflow)
• run all non-regression, non-example tests in optional.yml — previously only smoke tests ran, but it's probably better to be comprehensive
• fix test_modflow.py::test_pkg_data_containers
  • parametrized test IDs were not path-friendly on Windows, so could fail to create a temp dir named for the test case
  • the model could sometimes fail to converge, fix it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (86eb092) 73.0% compared to head (a5a3219) 71.2%.
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2089     +/-   ##
=========================================
- Coverage     73.0%   71.2%   -1.9%     
=========================================
  Files          259     259             
  Lines        59357   57980   -1377     
=========================================
- Hits         43352   41290   -2062     
- Misses       16005   16690    +685     

see 106 files with indirect coverage changes