Add Keck atmosphere from KAON303

[vkooten]

this adds the feature from issue #151

[codecov[bot]]

Codecov Report

Merging #152 (6922d23) into master (ce6b169) will decrease coverage by 0.09%. The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage   81.78%   81.70%   -0.09%     
==========================================
  Files          97       97              
  Lines        7319     7328       +9     
==========================================
+ Hits         5986     5987       +1     
- Misses       1333     1341       +8     

Files	Coverage Δ
hcipy/atmosphere/__init__.py	100.00% <ø> (ø)
hcipy/atmosphere/standard_atmosphere.py	72.41% <11.11%> (-27.59%)	:arrow_down:

:mega: Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[syhaffert]

I think something happened with the white spaces. Are you using the same indentation style as hcipy?

[ehpor]

@vkooten Taking this over to push this over the finish line.