Consolidation on use of pathlib module for filesystem operations, rather than pathlib & os.path

[charles-turner-1]

Currently, this package uses both pathlib.Path operations & os.path.join in a lot of places, with usage of both in plenty of files.

Pathlib has been the preferred way of doing things since Python 3.4 , and so we should think about moving over to using pathlib where possible.

Low priority.

[marc-white]

Also noting that a lot of functions either quietly expect a directory string, or a Path object, but will fail if the other is given. This should be consolidated to be a Path object everywhere.