set multiprocessing start method for MacOS

[aelanman]

Closes #47 .

Downloading kernel files fails on MacOS because astropy's downloader uses multiprocessing, which defaults to spawn on Macs vs. fork on linux. This ensures that the fork method is set once the package is imported.

This will not be needed once we've #46 , but fixes an issue for now.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.42%. Comparing base (7c0e88d) to head (794f903).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lunarsky/__init__.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   86.73%   86.42%   -0.31%     
==========================================
  Files           8        8              
  Lines         618      619       +1     
==========================================
- Hits          536      535       -1     
- Misses         82       84       +2     

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