test: refine pytest skip framework, improve confest.py

[mwtoews]

This PR builds on the new pytest framework with a few changes:

• add pytest_report_header to show packages installed (or not). For example, at the top of every pytest run shows something like this:

required packages: numpy-1.23.1, matplotlib-3.5.2
optional packages: pandas-1.4.3, pyshp-2.3.1, python-dateutil-2.8.2, shapely-1.8.2, vtk-9.1.0
optional packages not found: affine, appdirs, descartes, geojson, netcdf4, pyproj, scipy, xmipy, fiona, rasterio

• remove requires_exes, adjust requires_exe, which takes multiple (e.g.) @requires_exe("mf6", "gridgen")
• add has_exe, has_pkg, and requires_pkg functions
• use functions for tests, with a combination of installed packages (nothing too comprehensive, just a few combinations were tested)
• add run_cmd and run_py_script functions
• example notebooks and scripts can skip if they use missing optional dependencies
• update DEVELOPER.md with new changes
• and other small details, e.g. using git rev-parse --abbrev-ref HEAD to get branch name, etc.

[codecov]

Codecov Report

Merging #1486 (39b0138) into develop (861d556) will decrease coverage by 42.551%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           develop     #1486        +/-   ##
==============================================
- Coverage   72.493%   29.941%   -42.552%     
==============================================
  Files          250       250                
  Lines        54002     52532      -1470     
==============================================
- Hits         39148     15729     -23419     
- Misses       14854     36803     +21949     

Impacted Files	Coverage Δ
flopy/mfusg/mfusgsms.py	6.190% <0.000%> (-91.941%)	⬇️
flopy/mf6/utils/postprocessing.py	9.259% <0.000%> (-88.889%)	⬇️
flopy/seawat/swtvdf.py	8.092% <0.000%> (-88.460%)	⬇️
flopy/modflow/mfevt.py	7.537% <0.000%> (-87.463%)	⬇️
flopy/utils/mtlistfile.py	4.620% <0.000%> (-85.348%)	⬇️
flopy/mf6/utils/lakpak_utils.py	2.097% <0.000%> (-85.315%)	⬇️
flopy/modflow/mfhyd.py	13.223% <0.000%> (-85.124%)	⬇️
flopy/utils/swroutputfile.py	10.714% <0.000%> (-85.079%)	⬇️
flopy/modflow/mfag.py	7.282% <0.000%> (-83.878%)	⬇️
flopy/modflow/mfhob.py	5.923% <0.000%> (-83.005%)	⬇️
... and 205 more