GRASS-Addons: bulk PEP8 Python fixes on entire grass7/ subdirectory tree

Hi addon devs,

To improve the code quality, I have applied "flake8" recursively on
grass-addons/grass7/ and prepared a huge pull request with the
changes:

https://github.com/OSGeo/grass-addons/pull/357

In this pull request the fixes are separated by commit for easier review.
Eventually, after review, all will be squashed into one single commit.

Rationale: follow more closely our coding style:
https://trac.osgeo.org/grass/wiki/Submitting/Python#PEP8standardStyle

In total, 330 files will be changed - hence I recommend a timely
update of your local repository copy (`git pull`) once this is merged.

Comments are welcome in PR 357 (addons repo).

Markus