Add Mask subclass

[rhugonnet]

This PR:

• Adds the Mask class,
• Casts any boolean array passed to Raster.from_array() into a Mask,
• Makes __array_function__ interface compatible with logical functions, automatically cast into a Mask
• Changes output of Vector.create_mask() and rasterize() with certain parameters to Mask,
• Changes __eq__ into raster_equal().

Write new tests for all.

Resolves #316

[rhugonnet]

Ready for review, just need to make mypy happy and it's all done.

[rhugonnet]

Closing this, as it had to be developed directly in #351. See there for the final Mask features and tests.