Tidy up docstrings, add Level.is_observed property

[wtbarnes]

This PR actually renders the docstring of the function added in #320 and also tidies up docstrings all over the package, many of which have been missing for a long time.

The only feature this adds is the property Level.is_observed to denote whether the energy of a level is taken from laboratory measurements or theoretical calculations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.54%. Comparing base (a661c8c) to head (37fda76).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   90.51%   90.54%   +0.02%     
==========================================
  Files          40       40              
  Lines        3384     3394      +10     
==========================================
+ Hits         3063     3073      +10     
  Misses        321      321              

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