Add FY-3D MERSI-2 L1B Reader (mersi2_l1b)

[djhoese]

This PR adds a reader for the MERSI-2 L1B data format which are HDF5 files. This PR also includes some clean up of the VIRR reader (removing the "level" from the DatasetID).

TODO

• Closes Add reader for FY3D MERSI2 L1B data #706
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #740 into master will increase coverage by 0.27%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##           master     #740      +/-   ##
==========================================
+ Coverage   81.14%   81.41%   +0.27%     
==========================================
  Files         152      154       +2     
  Lines       22068    22427     +359     
==========================================
+ Hits        17908    18260     +352     
- Misses       4160     4167       +7

Impacted Files	Coverage Δ
satpy/scene.py	90.08% <ø> (ø)	⬆️
satpy/readers/hdf5_utils.py	92.3% <0%> (ø)	⬆️
satpy/tests/reader_tests/__init__.py	97.87% <100%> (+0.04%)	⬆️
satpy/tests/reader_tests/test_virr_l1b.py	97.16% <100%> (-0.03%)	⬇️
satpy/readers/virr_l1b.py	98.3% <87.5%> (+0.02%)	⬆️
satpy/readers/mersi2_l1b.py	96.29% <96.29%> (ø)
satpy/tests/reader_tests/test_mersi2_l1b.py	98.55% <98.55%> (ø)

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[coveralls]

Coverage increased (+0.3%) to 81.42% when pulling 1f67a3e on djhoese:feature-mersi2-reader into 1f05e0a on pytroll:master.

[mraspaud]

LGTM

[djhoese]

As discussed on slack, I've updated the reader so it preserves the uint16 data type when the calibration is "counts".