Allow for different default units

[PreisTo]

• created a set_units function to manually set the units used in the analysis (must be called before creating threeML and astromodels objects)
• added Units dataclass to the config instead of hard-coding the default units

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.79%. Comparing base (0dff91f) to head (e024393).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #239      +/-   ##
==========================================
+ Coverage   80.33%   82.79%   +2.46%     
==========================================
  Files          59       60       +1     
  Lines        8015     8051      +36     
==========================================
+ Hits         6439     6666     +227     
+ Misses       1576     1385     -191     

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