Expose bucket resampling from Pyresample

[pnuu]

This PR exposes the different bucket resampling methods from Pyresample to Satpy.

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

[pnuu]

Before merging pytroll/pyresample#192 needs to be merged first.

[pnuu]

And naturally the tests need to be re-run after merging the Pyresample PR.

[sfinkens]

Thanks for this nice piece of work! Just two comments.

[djhoese]

What was the reason for removing fraction resampling?

[pnuu]

After the suggestion by @djhoese I added the bucket_fraction resampler, which concatenates the resulting category fractions and adds a categories dimension.

[djhoese]

A couple questions and one small change. Otherwise, looks good.

[codecov]

Codecov Report

Merging #768 into master will increase coverage by 0.11%.
The diff coverage is 97.92%.

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
+ Coverage   85.16%   85.27%   +0.11%     
==========================================
  Files         174      174              
  Lines       25500    25741     +241     
==========================================
+ Hits        21716    21951     +235     
- Misses       3784     3790       +6

Impacted Files	Coverage Δ
satpy/tests/test_resample.py	98.32% <100%> (+0.6%)	⬆️
satpy/resample.py	92.94% <93.9%> (+0.17%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	90.14% <0%> (-1.41%)	⬇️

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[coveralls]

Coverage increased (+0.1%) to 85.276% when pulling b32428b on pnuu:feature-bucket-resampling into d1b47e0 on pytroll:master.