Add stages to interact with healpix maps and generating random coordinates

[jlvdb]

Feature request

Implement stages that implement

• healpix map arithmetic
• masking of catalogs based on coordinates
• healpix map creation based on coordinates and given nside resolution
• generating uniform randoms in RA/Dec limits
• generating uniform randoms efficiently on a healpix map (discrete randoms, draw random pixel center coordinates at maximum resolution of 536.870.912)
• resampling healpix maps to different resolutions

[sidneymau]

Noting that most, if not all, of these features are implemented in HealSparse -- for example, see https://healsparse.readthedocs.io/en/latest/randoms.html

[jlvdb]

Thank you for pointing that out, I didn't know the package before. No need to reinvent the wheel then.
The question is, if the proposed features are still of interest, especially as my time is very limited at the moment.