Update the solar spectrum code to use the included reference spectra

[djhoese]

Redo of #202 to get around permission issues. This also includes updated docs and tests to take advantage of the default argument added by @simonrp84.

• Closes #xxxx
• Tests added
• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fd32fe9) 89.27% compared to head (7830e7d) 90.00%.
Report is 30 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
+ Coverage   89.27%   90.00%   +0.73%     
==========================================
  Files          23       23              
  Lines        2545     2571      +26     
==========================================
+ Hits         2272     2314      +42     
+ Misses        273      257      -16     

Flag	Coverage Δ
unittests	90.00% <100.00%> (+0.73%)	⬆️

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