Finalize NIRCam spectra support, updates for STIS, etc.

[karllark]

Finalize NIRCam slitless capabilities. Both observed and mocked spectra.

Adding keyword to only read in certain bands (useful for SED modeling).

Adding in new STIS filenaming convention.

Other minor improvements.

[codecov]

Codecov Report

Attention: Patch coverage is 22.22222% with 105 lines in your changes missing coverage. Please review.

Project coverage is 40.10%. Comparing base (819b545) to head (553e852).
Report is 7 commits behind head on master.

Files	Patch %	Lines
measure_extinction/utils/merge_nircam_spec.py	15.62%	54 Missing ⚠️
measure_extinction/utils/merge_miri_ifu_spec.py	8.57%	32 Missing ⚠️
measure_extinction/stardata.py	58.33%	10 Missing ⚠️
measure_extinction/utils/plot_mspec.py	14.28%	6 Missing ⚠️
measure_extinction/utils/merge_stis_spec.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   41.04%   40.10%   -0.94%     
==========================================
  Files          21       22       +1     
  Lines        2863     2977     +114     
==========================================
+ Hits         1175     1194      +19     
- Misses       1688     1783      +95     

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