Enable testing on windows and macos platforms

[kyleaoman]

Closes #55
Depends on SWIFTSIM/swiftsimio#258

With better management of file opening/closing on the swiftsimio side, and using requests instead of wget, the swiftgalaxy test suite should be able to run on windows and macos platforms.

• Once the swiftsimio patch merges (now merged) and releases (not yet), reset the dependency to the pypi version. Then this can be merged if tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (17b2eb1) to head (ee870cc).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #67   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         2314      2314           
  Branches       265       265           
=========================================
  Hits          2314      2314           

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