Add a reader for NOAA GOES-R ABI L2+ products (abi_l2_nc)

[djhoese]

This is a replacement of #767. That PR was made with @yufeizhu600's master branch so I didn't have push permissions. This PR is the same as that one except I merged master in to it and did a few small docstring cleanups. I also removed the __init__ definition in the l2 reader module because it was unnecessary.

[coveralls]

Coverage increased (+0.2%) to 85.087% when pulling 9939a5c on djhoese:zufeizhu600-master into 1389229 on pytroll:master.

[codecov]

Codecov Report

Merging #904 into master will increase coverage by 0.24%.
The diff coverage is 85.52%.

@@            Coverage Diff             @@
##           master     #904      +/-   ##
==========================================
+ Coverage   84.84%   85.09%   +0.24%     
==========================================
  Files         171      174       +3     
  Lines       25297    25428     +131     
==========================================
+ Hits        21464    21637     +173     
+ Misses       3833     3791      -42

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_scmi.py	96.19% <100%> (ø)	⬆️
satpy/tests/reader_tests/__init__.py	98.18% <100%> (+0.06%)	⬆️
satpy/readers/abi_l2_nc.py	27.27% <27.27%> (ø)
satpy/tests/reader_tests/test_abi_l1b.py	96.36% <83.33%> (-0.86%)	⬇️
satpy/readers/abi_l1b.py	98.3% <88.88%> (+3.68%)	⬆️
satpy/readers/abi_base.py	90.82% <90.82%> (ø)
satpy/tests/reader_tests/test_abi_l2_nc.py	94.02% <94.02%> (ø)
... and 3 more

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