Make sunz correction use available sunz dataset

[mraspaud]

I noticed that the sun zenith angles being an optional dataset, it was never used by the SunZenithCorrector. This PR fixes the issue.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented

[ghost]

DeepCode's analysis on #3a29b0 found:

1 minor issue. ✔️ 1 issue were fixed.

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[codecov]

Codecov Report

Merging #1089 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1089      +/-   ##
==========================================
+ Coverage   89.43%   89.46%   +0.02%     
==========================================
  Files         198      198              
  Lines       29033    29167     +134     
==========================================
+ Hits        25965    26093     +128     
- Misses       3068     3074       +6     

Impacted Files	Coverage Δ
satpy/composites/__init__.py	78.37% <100.00%> (-0.30%)	⬇️
satpy/tests/test_composites.py	100.00% <100.00%> (ø)
satpy/readers/tropomi_l2.py	90.00% <0.00%> (-4.12%)	⬇️
satpy/tests/writer_tests/test_mitiff.py	99.10% <0.00%> (+0.13%)	⬆️
satpy/writers/mitiff.py	91.80% <0.00%> (+0.14%)	⬆️
satpy/writers/__init__.py	86.50% <0.00%> (+0.48%)	⬆️
satpy/tests/reader_tests/test_tropomi_l2.py	98.36% <0.00%> (+0.63%)	⬆️
satpy/config.py	83.47% <0.00%> (+1.32%)	⬆️
satpy/tests/test_config.py	91.13% <0.00%> (+1.58%)	⬆️

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[coveralls]

Coverage increased (+0.03%) to 89.461% when pulling 3a29b05 on mraspaud:fix-sunz-correction-computation into 7b71a90 on pytroll:master.