Cleanup project structure

[luleyleo]

Cleaning up the project structure before I rewrite the documentation for #96 .

[luleyleo]

@Saethox I think this makes MAHF's structure a bit more intuitive. If you have further suggestions, please let me know.

[Saethox]

Looks good to me, and some of these changes are much needed. I think we can merge after the CI passes.

And if we are already refactoring: the SimpleEvaluator should in my opinion also live in its own module, and be renamed to something like SequentialEvaluator. But I can handle this in #95, as I will have to touch the code anyway there.

On another note, I don't like the way the problem::tsp module is organized (coming from the person who wrote it), so I'd really like to refactor it, but this is really not the most pressing problem. But I will probably add an issue.

[Saethox]

I think this PR also closes #99.