Fix sunz_corrected modifier adding unnecessary x and y coordinates

[djhoese]

See #587 for details. This is a replacement for that PR.

In summary, the sunz_corrected modifier was adding new x and y coordinates to the SZA data it was creating. This happened because my_data_arr['y'] will create a new data array even if the 'y' coordinate doesn't exist. The value would be a range representing the indexes of the array. The way this was being used was adding y/x coordinates to the dataset when the original array didn't have them. This would then add these coordinates to the input data and pass it on to the DayNightCompositor. The DayNightCompositor would then get confused if it had y/x for one dataset and no coordinates for the other. This PR makes it so those coordinates are only added if they already existed in the input data to the sunz_corrected modifier.

• Closes Drop x and y indexes before zeroing data in DayNightCompositor #587, fixes DayNightCompositor does not work with eg overview_sun as the day part #584
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[djhoese]

I should have also mentioned this "fixes" (I think) the caching of the SZA data so that it is reused even if the SZA was provided by the reader.

[coveralls]

Coverage increased (+0.005%) to 77.584% when pulling bf4a79e on djhoese:bugfix-sunz-corrected-coords into 5c5f0c5 on pytroll:master.

[coveralls]

Coverage increased (+0.02%) to 77.595% when pulling 4377291 on djhoese:bugfix-sunz-corrected-coords into 5c5f0c5 on pytroll:master.

[codecov]

Codecov Report

Merging #593 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #593      +/-   ##
==========================================
+ Coverage   77.57%   77.59%   +0.01%     
==========================================
  Files         136      136              
  Lines       19179    19192      +13     
==========================================
+ Hits        14879    14892      +13     
  Misses       4300     4300

Impacted Files	Coverage Δ
satpy/composites/__init__.py	64.36% <100%> (+0.2%)	⬆️
satpy/tests/compositor_tests/__init__.py	99.15% <100%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5c5f0c5...4377291. Read the comment docs.

[pnuu]

Looks good to me, and works.