Move some general utils here

[teutoburg]

Three utils functions from ScopeSim that are general enough to be kept here, and also only depend on standard library stuff.

I didn't find any unit tests for those in ScopeSim, so I added some here. Also added some better docstrings.

Also added custom warning base classes inspired by astropy, to be used throughout our ecosystem in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.36%. Comparing base (0b3537d) to head (644d5fe).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   99.31%   99.36%   +0.05%     
==========================================
  Files           6        8       +2     
  Lines         435      472      +37     
==========================================
+ Hits          432      469      +37     
  Misses          3        3              

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[teutoburg]

This is a bad idea. check_keys() will become obsolete when we use dataclasses, stringify_dict() is smelly anyway, and close_loop() alone isn't worth moving here unless we actually need it in another package (which might still happen at some point). I might add the tests and docstrings I added here to the current location of those functions and also reuse the custom warnings in a separate PR, as those might actually be useful elsewhere.