Integrate flake8_rst into ./ci/code_check.sh
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Integrated [flake8-rst](https://github.com/kataev/flake8-rst) into `code_check.sh` in order to check code in .. code-blocks:: python directives

* fixed all issues found with version v0.4.1
    - F821 undefined name ignored  -- too many occurences for now
    - W391 blank line at end of file  -- 4 occurences where I had no idea how to fix

```
doc/source/contributing_docstring.rst:629:9: W391 blank line at end of file
doc/source/contributing_docstring.rst:725:9: W391 blank line at end of file
doc/source/contributing_docstring.rst:815:9: W391 blank line at end of file
doc/source/contributing_docstring.rst:911:9: W391 blank line at end of file
```


