Add 'reduce_data' keyword argument to disable cropping before resampling

[pnuu]

This PR adds a way to disable data reduction before resampling. This is a work-around for #383 that might be useful even after fixing pyresample.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

Usage when calling resampling:

lcl = glbl.resample(..., reduce_data=False)

The default is to apply data reduction.

Not sure where to document this, as resampling documentation is coming direct from resample.py docstrings, and this fix happens in scene.py. Suggestions?

[coveralls]

Coverage increased (+0.01%) to 77.437% when pulling 1fa2dcc on pnuu:feature-optional-datareduction into 8288d4b on pytroll:master.

[coveralls]

Coverage increased (+0.1%) to 77.677% when pulling 3a32193 on pnuu:feature-optional-datareduction into 02d9e91 on pytroll:master.

[codecov]

Codecov Report

Merging #582 into master will increase coverage by 0.1%.
The diff coverage is 68.18%.

@@            Coverage Diff            @@
##           master     #582     +/-   ##
=========================================
+ Coverage   77.57%   77.67%   +0.1%     
=========================================
  Files         136      136             
  Lines       19194    19209     +15     
=========================================
+ Hits        14889    14921     +32     
+ Misses       4305     4288     -17

Impacted Files	Coverage Δ
satpy/tests/test_scene.py	99.42% <100%> (+0.1%)	⬆️
satpy/scene.py	86.77% <41.66%> (+2.64%)	⬆️
satpy/readers/__init__.py	94.46% <0%> (+1.38%)	⬆️

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[djhoese]

Could you explicitly add the keyword argument to the resample method and document it in the docstring. Otherwise this looks good to me.

[pnuu]

The test failures in generic_image reader seem to be related to version changes of either gdal, pyproj or rasterio. Locally the tests passed with gdal=2.3.2, pyproj=1.9.5.1 and rasterio=1.0.9 and fail after upgrading to latest packages.

[djhoese]

I merged #595 which should fix the errors you were receiving for now. I restarted the tests and hopefully it picks up on the changes in master. Otherwise I'll merge master in to your branch for this.