Add SpectralType class

[teutoburg]

Who needed that now??

Weeeell, technically one of our students would have benefited from this a little while back. More to the point, this will come in handy for scopesim-targets (maybe even for ScopeSim_Templates) and Pyckles and maybe also SpeXtra. Also, it took just this afternoon from initial idea to implementation including all 77 test cases, so why not 🤷‍♂️

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.26%. Comparing base (ff7168e) to head (c165603).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   99.16%   99.26%   +0.09%     
==========================================
  Files           5        6       +1     
  Lines         361      409      +48     
==========================================
+ Hits          358      406      +48     
  Misses          3        3              

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[hugobuddel]

O be a fine guy, kindly merge