Fix repeated masking

[kyleaoman]

There could be quite confusing indirection when unevaluated LazyMask objects got copied to new SWIFTGalaxy objects with e.g. sg[...]. The LazyMask would retain a reference to the original SWIFTGalaxy which could lead to evaluating the mask using incorrect data.

At the cost of now requiring an argument to LazyMask.evaluate and LazyMask.mask becoming a 1-argument function instead of a @property, a reference to the relevant SWIFTGalaxy is now available at mask evaluation time which should avoid this kind of issue.

Closes #89

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bf281ac) to head (caf651d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #90   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            7         7           
  Lines         2288      2287    -1     
  Branches       261       261           
=========================================
- Hits          2288      2287    -1     

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