[NEW FEATURE] Create coverage masks for UNIONS

[martinkilbinger]

A coverage mask is a required product to go along the UNIONS shape catalogue. It encodes the coverage by exposures, in healsparse format. It can be queried with ra, dec positions to return the number of exposures at that sky position.

Scripts and jupyer notebooks exist, courtesy Mike Hudson. They need to be included as library classes in this PR. Executable python scripts needs to be created on installation from pyproject.toml.

[cailmdaley]

@AstroMike could you please take a look at whether the linked PR #797 has the appropriate functionality or whether we need to import anything else from your scripts? if the latter, could you please link them? thank you!