Should mappers be recovered from map metadata?

[ntessore]

Currently, the transform() function requires fields and maps. However, the fields are only required to recover the mapper for each map, and call its .transform() method.

Alternatively, we could return to a system where the mappers can be reconstructed from the metadata of each map. The main reason this was abandoned was that it made it harder to use Heracles with external maps which don't have the required metadata. However, the metadata for external maps needs to be updated anyway even now (#201).

[ntessore]

To work with external maps, I could imagine a convenience function like the following:

mapper = heracles.healpy.HealpixMapper(nside, lmax)

m = hp.read_map("external_map.fits")
mapper.set_for_map(m, spin=2)

This could also check that the map is compatible with the given mapper.