Ensure point_map is written in IMPLICIT FULLSKY

[camposandro]

The point_map.fits sky map is currently written using HEALPy, in the IMPLICIT FULLSKY NESTED format. We should make sure that, after migrating away from HEALPy, we keep it this way. When it comes to that, we should use the methods to read/write sky maps of the CDS HEALPix Rust library (for which we might need interfaces in python).

[nevencaplar]

@camposandro Is this something that we want to have in before the do renaming HiPSCaT -> HATS

[camposandro]

I think we'll only need to worry about this when migrating away from HEALPy. The renaming shouldn't affect it.

[delucchi-cmu]

I would like to include this in the renaming, as we're re-writing most root-level files, and performing dataset conversions in batch. This may be a simple as ensuring that our arguments are:

            hp.write_map(
                <file name>, <map_fits_image>, dtype=np.int32, nest=True, coord="CEL"
            )