Delay mask evaluation more

[kyleaoman]

This PR (re-)streamlines accessing the mask attribute of a LazyMask so that it no longer requires an argument and can be a @property. It then adds features in masks.py to allow lazily combining two MaskCollections via lazy merging of LazyMasks, and significantly reduces the number of references to SWIFTGalaxy objects that we need to keep around attached to masks. LazyMask and MaskCollection now mostly don't need to know about SWIFTGalaxy, except to retrieve a particle count from the metadata when combining masks, which now just asks for the relevant SWIFTGalaxy at the relevant time.

Tests have been fleshed out and confirm that we can now delay evaluating masks (and therefore reading any data needed to evaluate them) indefinitely - only when a data read of the relevant particle type is triggered is the mask actually evaluated. Previously we could trigger reading data to evaluate masks for all types when applying a second mask because the masks could not be combined lazily. The implementation is also such that we don't end up with an insane function stack when we combine masks, just a chain of masks applied to np.arange(num_part).

Closes #79

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9a4eb5a) to head (258a1dd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            7         7           
  Lines         2287      2319   +32     
  Branches       261       261           
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+ Hits          2287      2319   +32     

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