Refactor emissivity calculations

[wtbarnes]

This PR does several things:

• Refactor emissivity calculations in EmissionModel
• Add continuum emission to narrowband emissivity (used in AIA intensity calculation)
• Speed up loading of HYDRAD results
• Refactor loading ionization fractions
• Add optional mask in field-aligned coordinate for chromospheric emission

[codecov-commenter]

Codecov Report

❌ Patch coverage is 29.41176% with 120 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.67%. Comparing base (ae369d2) to head (2a61e52).

Files with missing lines	Patch %	Lines
synthesizAR/atomic/emission_models.py	18.36%	80 Missing ⚠️
synthesizAR/interfaces/hydrad/hydrad.py	13.33%	13 Missing ⚠️
synthesizAR/instruments/sdo.py	42.10%	11 Missing ⚠️
synthesizAR/strand.py	28.57%	10 Missing ⚠️
synthesizAR/skeleton.py	69.23%	4 Missing ⚠️
synthesizAR/instruments/base.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   61.00%   61.67%   +0.67%     
==========================================
  Files          43       43              
  Lines        2226     2239      +13     
==========================================
+ Hits         1358     1381      +23     
+ Misses        868      858      -10     

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