Add two luminance sharpening compositors

[pnuu]

This PR adds two compositors that can be used to apply luminance from a VIS channel to RGB composites.

1. LuminanceSharpeninCompositor: replaces the luminance via RGB -> YCbCr -> RGB conversion
2. SandwichCompositor: multiplies the RGB channels with the reflectance (scaled to [0.0, 1.0])

Example composites for SEVIRI included are: vis_sharpened_ir and ir_sandwich

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

[coveralls]

Coverage decreased (-0.2%) to 74.094% when pulling 88340bc on feature-luminance-compositor into 6fc2669 on master.

[coveralls]

Coverage increased (+0.1%) to 74.569% when pulling 75800da on feature-luminance-compositor into 5dc74d2 on master.

[codecov]

Codecov Report

Merging #503 into master will increase coverage by 0.08%.
The diff coverage is 97.18%.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
+ Coverage   74.48%   74.56%   +0.08%     
==========================================
  Files         136      136              
  Lines       18328    18399      +71     
==========================================
+ Hits        13651    13720      +69     
- Misses       4677     4679       +2

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/__init__.py	98.45% <100%> (+0.35%)	⬆️
satpy/composites/__init__.py	43.86% <94.28%> (+2.55%)	⬆️

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[pnuu]

Still need to add tests, and start using the luminance conversion formulae from trollilmage instead of the ones copied here. The formulae seem to be to-and-from YUV, not YCbCr, so have to take a deeper look. But at least they produce reasonable results, so not yet sure what to do in the end.

[mraspaud]

Looks good, but I would rather reuse the functions from trollimage (which is a dependency of satpy) instead of copying them to satpy

[djhoese]

Agree with not copying functions that are supposed to exist in trollimage. FYI @pnuu there is this PR by @katherinekolman addressing the dask-ification of the convert method: pytroll/trollimage#28

I was thinking that she didn't need to implement the YUV/YCbCr conversions right now and would merge what she has since we don't need those other conversions (@mraspaud thoughts?). @pnuu You could add on to what she has started.

[pnuu]

Yes, I agree that eventually the conversions should go via the .convert() method, but for now I'm going with the rgb2ycbcr and ycbcr2rgb functions from trollimage.image. If I started to add the functionality to pytroll/trollimage#28 I'd be dependent on another branch and testing would fail. I think the convert() (with YCbCr) stuff should be available in released trollimage before swapping these compositors to use the functionality.