Refactor and initial tests for backuptool.

### Motivation
Bringing side modules up to scratch. Most of this was actually done a long time ago.

### Changelog
* Various little code and documentation changes; see the commit message for details.
* Separation of code from tests.

### Note
This is not urgent at all, but nicely showcases some of the python idioms. Have a look.
and then you have to return it here too?

I like the old plain approach better, which feels like a static method in any other language.
But oh well.