DOC: Adding script to validate docstrings, and generate list of all functions/methods with state
- [ ] closes #xxxx
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Adding a new script to check whether a docstring follows the defined standards or not. Also, without arguments, the script returns a report of all the functions and methods in the API, with information about its docstring.

CC @jorisvandenbossche 