Fix compact viirs angle interpolation at the poles

[mraspaud]

This PR fixes the wrong interpolation done at the poles for satellite and sun angles in the compact viirs reader.

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

The effect was very local but led to a dramatic effect on some composites. The following figure exhibits the problem affecting the sun zenith:

It turns out it was probably due to some numerical instability. This is addressed now in the cartesian to angles conversion

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@34e4045). Click here to learn what that means.
The diff coverage is 51.51%.

@@            Coverage Diff            @@
##             master     #901   +/-   ##
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  Coverage          ?   85.12%           
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  Files             ?      174           
  Lines             ?    25525           
  Branches          ?        0           
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  Hits              ?    21727           
  Misses            ?     3798           
  Partials          ?        0

Impacted Files	Coverage Δ
satpy/tests/test_utils.py	98.84% <100%> (ø)
satpy/utils.py	70.76% <100%> (ø)
satpy/readers/viirs_compact.py	15.04% <11.11%> (ø)

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