First round of mask refactor.

Mask.py fleshed out to have constructors from array, SpectralCube, and FITS. CASA to be implemented.

Mask exposed in a number of ways (array, indices, twod, attach to cube, etc.).

Basic undo functionality.

I don't think it should inherit SpectralCube but there's still some weirdness with needing linked_data for WCS. Will think.

PS Promote me to collaborator!