Switch to using pathlib everywhere

[johentsch]

Currently all paths are (to be) specified as strings and manipulated using os.path. Part of the strategy to do the switch is therefore to look for all uses of os.path.

• For aesthetics, whenever paths are displayed that are beneath the home directory, the latter should be replaced by ~ (as is currently the case with dimcat.utils.AbsolutePathStr).
• The serialization of all basepath (and other path) arguments needs to be adapted because Marshmallow does not recognize a Path object as string (create a custom field for paths, there is a marshmallow issue with solutions somewhere)