Disallow function redefinition (E0102).

### Motivation:
Redefined functions can hamper code readability and can lead to subtle errors. When intentional, they are an indicator of bad design. Let's rid ourselves of them.

### Changelog:
* Enable `pylint` redefinition check: `E0102`.
* Fix all present function redefinitions via renaming.
Not sure if we want to do BDD, but if so, this is very cumbersome. Perhaps we should disable E102 within the `features` directory instead? Basically this adds one unnecessary comment line for every behave step, and this is only to compensate for pylint not working well with behave...