Fix sphinx docs generation after setuptools_scm migration

[djhoese]

While working on #586 I noticed that the sphinx docs had a lot of unnecessary warnings. This PR fixes those warnings and all flake8 docstring issues that occurred in the files that needed to be changed.

Note that even with these changes you get these warnings from sphinx:

WARNING: autodoc: failed to import module 'grib' from module 'satpy.readers'; the following exception was raised:
No module named 'pygrib'
WARNING: autodoc: failed to import module 'hsaf_grib' from module 'satpy.readers'; the following exception was raised:
No module named 'pygrib'
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.hrit_base, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.seviri_l1b_hrit, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.hrit_jma, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.goes_imager_hrit, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.electrol_hrit, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.modis_l1b, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/readers.rst:2: WARNING: duplicate object description of satpy.readers.modis_l2, other instance in api/satpy.readers, use :noindex: for one of them
/home/davidh/repos/git/satpy/doc/source/resample.rst:2: WARNING: duplicate object description of satpy.resample, other instance in api/satpy, use :noindex: for one of them

Obviously the top two are from me not having pygrib installed, but the others are from us having readers (and resampling) re-documented in the readers sphinx page and the resample sphinx page. I know the readers were something that @mraspaud wanted because it provides a nice list and summary of the readers, but this seems to be confusing sphinx. Maybe this will be fixed when we think about auto-generating a reader table based on the metadata in the reader YAML file.

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

[coveralls]

Coverage remained the same at 85.41% when pulling ad30349 on djhoese:doc-fix-bad-formatting into a6ce4df on pytroll:master.

[codecov]

Codecov Report

Merging #943 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #943   +/-   ##
=======================================
  Coverage   85.41%   85.41%           
=======================================
  Files         172      172           
  Lines       26046    26046           
=======================================
  Hits        22246    22246           
  Misses       3800     3800

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_hrit.py	92.3% <100%> (ø)	⬆️
satpy/readers/modis_l2.py	98.52% <100%> (ø)	⬆️

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[sfinkens]

LGTM, build html doesn't print any warnings 👍 I also double checked the completeness of the reader list in readers.rst.

[sjoro]

no expertise on this matter, i'm happy to leave this on other reviewers.