Fix generation of solar and satellite angles when lon/lats are invalid

[djhoese]

This fixes the angle generation for the various compositors/modifiers that use the get_angles() functions so they actually check that the lon/lat degrees are valid. This should have been done for a long time since 1e30s were getting through all of the processing. Now in pyproj 2.4.2 these invalid values are infinity and this was causing some tests to fail.

This should probably have more tests added.

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

[coveralls]

Coverage increased (+0.01%) to 86.877% when pulling bd8619e on djhoese:debug-travis into 177d150 on pytroll:master.

[codecov]

Codecov Report

Merging #989 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #989      +/-   ##
==========================================
+ Coverage   86.86%   86.87%   +0.01%     
==========================================
  Files         181      181              
  Lines       27461    27487      +26     
==========================================
+ Hits        23854    23880      +26     
  Misses       3607     3607

Impacted Files	Coverage Δ
satpy/composites/__init__.py	78.36% <100%> (+0.05%)	⬆️
satpy/tests/compositor_tests/__init__.py	99.58% <100%> (ø)	⬆️
satpy/composites/viirs.py	83.85% <100%> (+0.07%)	⬆️
satpy/tests/compositor_tests/test_viirs.py	99.06% <100%> (+0.35%)	⬆️
satpy/scene.py	90.35% <0%> (-0.18%)	⬇️

---

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 177d150...bd8619e. Read the comment docs.

[mraspaud]

LGTM, I just have some more abstract questions