Adds use of GCPDefinition instead of SwathDefinition to Modis L1b reader

[BENR0]

These changes introduce the use of GCPDefinition (SwathDefinition with ground control points pytroll/pyresample#476) for the Modis L1b reader.

This should be used as a base for discussion of how/where SwathDefinitions/ GCPDefinitions should be created reader/FileYAMLReader. Currently if get_area_def is not implemented in the reader a SwathDefinition is created in the FileYAMLReader as a "fallback" in a kind of way.

Possible Questions:

• change get_area_def to get_area in the reader to make it clearer that this should be used for creation of AreaDefinitions as well as SwathDefinitions (with control points).
• refactor the logic of FileYAMLReader and readers how to create SwathDefinitions with ground control points and interpolation?
  • e.g. should it be necessary to include tiepoints as a variable and add it to the coordinates keyword for the other datasets in the yaml file of the reader (as is done here)? Alternatively introduction of a "depends_on"/"requires" keyword which can be used to indicate that full lat/lons depend on tiepoints?

...

• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2283 (71afcda) into main (f9c2070) will decrease coverage by 0.04%.
The diff coverage is 13.04%.

@@            Coverage Diff             @@
##             main    #2283      +/-   ##
==========================================
- Coverage   94.32%   94.28%   -0.05%     
==========================================
  Files         308      308              
  Lines       46168    46191      +23     
==========================================
+ Hits        43546    43549       +3     
- Misses       2622     2642      +20     

Flag	Coverage Δ
behaviourtests	4.61% <4.34%> (-0.01%)	⬇️
unittests	94.91% <13.04%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/yaml_reader.py	95.36% <9.52%> (-2.15%)	⬇️
satpy/readers/hdfeos_base.py	91.93% <50.00%> (-0.35%)	⬇️

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[coveralls]

Coverage decreased (-0.04%) to 94.867% when pulling 71afcda on BENR0:feature_tiepoints_def_modis into f9c2070 on pytroll:main.