Reader for Hydrology SAF precipitation products

[simonrp84]

This PR adds support for the EUMETSAT Hydrology SAF precipitation products. Specifically, the reader functions with the h03 and h03B products (instantaneous precipitation) and the h05 and h05B products (accumulated precipitation).
Other H-SAF products are not supported at this time.

An example: H-SAF instantaneous precipitation (h03B product) for Kenya.

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

[coveralls]

Coverage decreased (-0.2%) to 82.808% when pulling 0900b80 on simonrp84:h_saf into 8237e1b on pytroll:master.

[coveralls]

Coverage increased (+0.03%) to 83.63% when pulling ed6cacf on simonrp84:h_saf into fa0e046 on pytroll:master.

[codecov]

Codecov Report

Merging #814 into master will increase coverage by 0.03%.
The diff coverage is 87.57%.

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
+ Coverage   83.59%   83.63%   +0.03%     
==========================================
  Files         165      167       +2     
  Lines       24313    24490     +177     
==========================================
+ Hits        20325    20482     +157     
- Misses       3988     4008      +20

Impacted Files	Coverage Δ
satpy/tests/reader_tests/__init__.py	98.07% <100%> (+0.03%)	⬆️
satpy/readers/hsaf_grib.py	83.33% <83.33%> (ø)
satpy/tests/reader_tests/test_hsaf_grib.py	91.3% <91.3%> (ø)
satpy/scene.py	90.19% <0%> (+0.17%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	91.54% <0%> (+1.4%)	⬆️

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[mraspaud]

Thanks for adding hsaf products to satpy ! A couple of authors should be removed from the header. Also, I think we should rename the reader hsaf_grib for completeness.
Last thing for now, maybe pygrib should be added to setup.py, in the extra dependencies ?

[mraspaud]

@simonrp84 could you add a couple of images to the PR description for completeness and documentation ?

[mraspaud]

Almost there !