support for departure coefficients

[ajwheeler]

This PR implements support for departure coefficients, and includes an algorithm for matching levels from the linelist to the model atom which was suggested by Thomas Nordlander.

TODO

• tests
• documentation
• joint atmosphere and departure coef interpolation
• custom Korg departure coef format?
• automatic downloads
• understand comparison to SME for GALAH spectra. (Why are line depths slightly different?)
• ensure no performance regression for pure-LTE synthesis

[codecov]

Codecov Report

Attention: Patch coverage is 43.52941% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 88.56%. Comparing base (1105e8c) to head (f75ec80).
Report is 8 commits behind head on main.

❗ Current head f75ec80 differs from pull request most recent head 0bce89e. Consider uploading reports for the commit 0bce89e to get more accurate results

Files	Patch %	Lines
src/levels.jl	0.00%	31 Missing ⚠️
src/line_absorption.jl	61.53%	10 Missing ⚠️
src/synthesize.jl	75.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
- Coverage   89.95%   88.56%   -1.40%     
==========================================
  Files          27       27              
  Lines        2130     2142      +12     
==========================================
- Hits         1916     1897      -19     
- Misses        214      245      +31     

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