convert: using spaces instead of tabs now

yes, we finally gave up being rebellious, and adapted
another pep8 guideline.

you may still use tabs transparently on your local machine,
git can automagically convert the spaces to tabs for your editing.

=> your files will have tabs, but git stores them as spaces.

in your local repo, create/edit .git/info/attributes:

	*.py  filter=tabspace

then configure the repository:

	git config --global filter.tabspace.smudge 'unexpand --tabs=4 --first-only'
	git config --global filter.tabspace.clean  'expand --tabs=4 --initial'

update all files in the local repo to have tabs instead of spaces:

	git checkout HEAD -- **

when you commit and push your code, the tabs will be saved as spaces.
doc: added tabs-vs-spaces guideline file
ci: added coverty static analysis integration