Enhanced masking

[lboehme]

Two additions to the masking functionality would be really useful:

• Allow masks which define the included - instead of the excluded - regions, by giving some additional argument in the .json parser like inverse_mask: True
• Give the possibility to directly import binary masks prepared beforehand which fit to the given Nside. This would allow the usage of more complex masks, which can not be defined by simple RA, Dec bounds.

[JonahDW]

Latest commit should have addressed both of these points, can you check if this fits your requirements?