Add CALSPEC model solar spectrum

[mkelley]

Adding support for a R=5000 model solar spectrum from STScI's CALSPEC database.

[mkelley]

Requires #370 .

[codecov]

Codecov Report

Merging #371 (4ba3da1) into main (8950548) will decrease coverage by 0.04%.
The diff coverage is 77.50%.

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
- Coverage   76.10%   76.06%   -0.04%     
==========================================
  Files          78       78              
  Lines        6921     6928       +7     
==========================================
+ Hits         5267     5270       +3     
- Misses       1654     1658       +4     

Impacted Files	Coverage Δ
sbpy/calib/tests/test_sun.py	87.03% <76.92%> (-3.16%)	⬇️
sbpy/calib/solar_sources.py	100.00% <100.00%> (ø)

[mkelley]

The decrease in coverage is due to adding a remote test. So, this is ready for review.

[jianyangli]

Just curious: In your recent few PRs, you are changing single quote (') to double quote ("). Is that something new in Python 3.10 or 3.11? I'm still using 3.8, but wondering whether I need to do the same to my personal code if I upgrade. Thanks.

[mkelley]

It isn't important, just a style preference. I like to use the Black formatter and it automatically does the change "The main reason to standardize on a single form of quotes is aesthetics. Having one kind of quotes everywhere reduces reader distraction." https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#strings