Fix enhance2dataset to support P-mode datasets

[mraspaud]

Allows P-datasets to be converted to RGB during the BackgroundCompositor operations.

• Depends on Fix infer_mode not using the band coordinate #1430
• Tests added

[ghost]

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[codecov]

Codecov Report

Merging #1432 (bbd6b6f) into master (cafa071) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1432      +/-   ##
==========================================
+ Coverage   90.64%   90.66%   +0.01%     
==========================================
  Files         236      236              
  Lines       33864    33931      +67     
==========================================
+ Hits        30695    30762      +67     
  Misses       3169     3169              

Impacted Files	Coverage Δ
satpy/composites/__init__.py	87.81% <100.00%> (+0.29%)	⬆️
satpy/tests/test_composites.py	99.87% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Coverage increased (+0.05%) to 90.66% when pulling bbd6b6f on mraspaud:feature-enhance2dataset-p into cafa071 on pytroll:master.