Getting value error when using dll interface with the cadet-core pip installation

[hannahlanzrath]

When installing cadet-core from pipy instead of conda, we get a

ValueError: Set Cadet to use_dll but no dll interface found.

This is since the python pypi package lives at a different place than the conda package. I fixed the issue for the general autodetect, but it has to be specified for the dll interface as well.