Proposal for a FITS convention for region based "skymaps"

[adonath]

In Gammapy we already use the GADF sky maps formats for HEALPix and WCS. In Gammapy there is now the use-case to handle "spectral" data, that was integrated / averaged over a certain region with additional extra axes, e.g. time.

This pull request proposes a new format for these "region based" maps. Here is the core of the proposal:

• Store the definition of the axes in the same "Bands HDU" as defined here: https://gamma-astro-data-formats.readthedocs.io/en/latest/skymaps/index.html#bands-hdu
• Allow for an additional "DATA" column in this HDU to store the data
• Allow for an optional region HDU, storing the region definition in the standard FITS convention described here: https://fits.gsfc.nasa.gov/registry/region.html

I hope the general need for such as format is uncontroversial, as it will allow to store e.g. time dependent counts spectra together with complex region definitions. Already now there are many use-cases where this format would be useful.

Currently the description is rather short, but as noted above the proposed format relies almost exclusively on already defined HDU definitions.