Fix EWA resampling tests not properly testing caching

[djhoese]

This was first noticed by @zxdawn in #751. Starting with dask 2.0+ these tests failed. The main reason is that this test was not testing what it was supposed to be testing after changes a long time ago to resample.py. The other reason is that in dask 2.0+, any map_blocks function is called with fake/empty input data to check what array type object is going to be returned. This meant multiple calls to ll2cr when previously there were only calls for actual data calculations.

So this PR fixes:

1. The EWA tests so that they actually make sure things were cached.
2. Resampler caching so that hashing of resample_kwargs results in proper equality checks, meaning resampler's that should be re-used are reused.
3. Fix geoviews conversion methods only issuing a warning if geoviews was not imported (FYI @BENR0) even though nothing could be done if it failed to import.

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

[djhoese]

FYI See dask/dask#5007 for my questions/concerns about this dask metadata discovery.

[codecov]

Codecov Report

Merging #831 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #831      +/-   ##
=========================================
+ Coverage   83.84%   83.9%   +0.05%     
=========================================
  Files         167     167              
  Lines       24441   24445       +4     
=========================================
+ Hits        20492   20510      +18     
+ Misses       3949    3935      -14

Impacted Files	Coverage Δ
satpy/scene.py	90.65% <100%> (+0.63%)	⬆️
satpy/tests/test_resample.py	85.89% <100%> (+0.37%)	⬆️
satpy/resample.py	89.38% <100%> (+2.15%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	90.14% <0%> (-1.41%)	⬇️
satpy/composites/__init__.py	74.64% <0%> (+0.25%)	⬆️

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[coveralls]

Coverage increased (+0.06%) to 83.903% when pulling bfeacd0 on djhoese:bugfix-ewa-tests into fa6f33e on pytroll:master.

[mraspaud]

Lgtm