Allow on-the-fly decompression of xRIT files in xRIT readers

[mraspaud]

This PR add on-the-fly decompression for xRIT files if the XRIT_DECOMPRESS_PATH environment variable is specified.

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

[coveralls]

Coverage increased (+1.6%) to 74.355% when pulling d03dd73 on feature-hrit-decompress into 1d3b128 on master.

[codecov]

Codecov Report

Merging #436 into master will decrease coverage by 1.53%.
The diff coverage is 84.09%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
- Coverage   74.31%   72.78%   -1.54%     
==========================================
  Files         136      131       -5     
  Lines       18188    17547     -641     
==========================================
- Hits        13517    12771     -746     
- Misses       4671     4776     +105

Impacted Files	Coverage Δ
satpy/readers/hrit_electrol.py	34.42% <100%> (ø)	⬆️
satpy/readers/hrit_goes.py	51.65% <100%> (ø)	⬆️
satpy/readers/hrit_msg.py	21.46% <100%> (-23.17%)	⬇️
satpy/readers/hrit_jma.py	97.67% <100%> (ø)	⬆️
satpy/readers/hrit_base.py	79.5% <76%> (-1.92%)	⬇️
satpy/tests/reader_tests/test_hrit_base.py	93.87% <94.11%> (+0.03%)	⬆️
satpy/readers/nc_olci.py	22.72% <0%> (-19.81%)	⬇️
satpy/readers/nc_nwcsaf.py	17.02% <0%> (-19.17%)	⬇️
satpy/composites/__init__.py	32.78% <0%> (-8.37%)	⬇️
... and 52 more

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[djhoese]

I'm confused. You removed the pattern so compressed files aren't going to be matched anymore, right?