Interactive updating of SkycalcTERCurve

[oczoske]

SkycalcTERCurve gains a prettier __str__ and can be updated on the fly with, e.g.,

metis['skycalc_atmosphere'].update(pwv=20, airmass=3)

(the name of the branch is somewhat misleading as there is only a small change to AtmoLibraryTERCurve)

[codecov]

Codecov Report

❌ Patch coverage is 67.44186% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.87%. Comparing base (158b4d2) to head (c161011).

Files with missing lines	Patch %	Lines
scopesim/effects/ter_curves.py	53.57%	13 Missing ⚠️
scopesim/source/source_fields.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
- Coverage   74.94%   74.87%   -0.07%     
==========================================
  Files          69       69              
  Lines        8843     8863      +20     
==========================================
+ Hits         6627     6636       +9     
- Misses       2216     2227      +11     

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[teutoburg]

Nothing jumped out at me as potentially problematic, so I assume it's good. Does have tests after all. "prevent[ion of] mental anguish" is always welcome 👍