Use either only Paths or only Strings not both

[schewskone]

I think we sometimes use strings instead of Path objects. We should settle on one of the two, probably Paths.
@pollytur @reneburghardt any opinions?

[pollytur]

i dont really have a strong opinion on this tbh. Either is fine for me
Maybe could you link a couple of code lines where its inconsistent / why is it an issue?

[schewskone]

I'm closing this. I scanned the interpolator file and it's minor function API's that are wrong. I'm adressing this in #81