feat(csub): add z-displacement support for disu grids (#1358)

* add function to calculate the area factor in base discretization
* use gridgen available from targets
* add pyshp testing dependency for flopy gridgen functionality
* add shapely testing dependency for flopy gridgen functionality
* add comparison of z-displacement and compaction for dis, disc, and unrefined disu
* add comparison of z-displacement and compaction for refined disu
* create reusable mapping dictionary for refined disu grids in eval_zdis
* revert changes to mf6core.vfproj
* rename variable m->idx_conn