Fix handling of modifiers in satpy-cf reader

[sfinkens]

Coordinates can have a modifiers attribute, too.

• Tests added
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #1447 (1ce23a5) into master (bfc7bfb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1447   +/-   ##
=======================================
  Coverage   90.52%   90.52%           
=======================================
  Files         238      238           
  Lines       34152    34153    +1     
=======================================
+ Hits        30915    30916    +1     
  Misses       3237     3237           

Flag	Coverage Δ
behaviourtests	4.52% <0.00%> (-0.01%)	⬇️
unittests	91.00% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_satpy_cf_nc.py	100.00% <ø> (ø)
satpy/readers/satpy_cf_nc.py	98.57% <100.00%> (+0.02%)	⬆️

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[mraspaud]

LGTM