ENH: Standardize 2.5D => Z in geometry type labels

[brendan-ward]

Resolves #225

This standardizes from 2.5D <type> to <type> Z for geometry type labels.

[jorisvandenbossche]

Looks good!

Do we want to keep some test with the 2.5D version to ensure this keeps working as well? (it's difficult to see from the diff if that's the case)

[brendan-ward]

We have a test test_geopandas_io.py::test_write_geometry_z_types that tests setting the geometry type when writing a dataset that uses both 2.5D and Z variants (didn't need to be updated here). I think that tests the only case where geometry type is an input.