Add utility function to compute the relative azimuth angle.

[simonrp84]

This PR adds a utility function to calculate the relative azimuth angle between sun and sensor. This code already existed in the PSPRayleighReflectance class, but here I move it to be a function in modifiers.angles.py so that users can access it more easily (as they can with the get_angles function).

Also, I have moved a lot of the angles tests from satpy/tests/test_modifiers.py into satpy/tests/modifier_tests/test_angles.py.

• [x] Tests added
• [x] Fully documented

[codecov[bot]]

Codecov Report

Merging #2175 (377283d) into main (fcbc517) will increase coverage by 0.00%. The diff coverage is 81.25%.

@@           Coverage Diff           @@
##             main    #2175   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files         300      300           
  Lines       45638    45649   +11     
=======================================
+ Hits        43014    43026   +12     
+ Misses       2624     2623    -1     

Flag	Coverage Δ
behaviourtests	4.66% <12.50%> (+<0.01%)	:arrow_up:
unittests	94.89% <81.25%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/modifiers/atmosphere.py	46.26% <25.00%> (+0.68%)	:arrow_up:
satpy/modifiers/angles.py	99.16% <100.00%> (+0.01%)	:arrow_up:
satpy/tests/modifier_tests/test_angles.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.003%) to 94.847% when pulling 377283d69b18c57d7326cdcf4d8411c4b5186e7b on simonrp84:raa_calc into fcbc517578c7b8a3e206ec9b48d2f1cb4239ff87 on pytroll:main.

[ghiggi]

Can I be of any help to make this merged @simonrp84? Would you be in favor of adding utility functions to compute also the phase angle and a sun glint angle or flag ?

[simonrp84]

Thanks for the offer @ghiggi, I hope to get this PR ready today - phase_angle and sun_glint_angle would also be really useful, perhaps you could make a separate PR for that once this one is merged? Happy to help code / review it!