Make function signatures more consistent

[schlegelp]

navis has grown organically and I have spent only minimal thought on keeping things consistent across functions. For the next major version it would be nice if we could try levelling the playing field somewhat by making sure parameters with similar meaning always have the same name.

Some examples (will expand the list as I spot cases):

• functions use output (e.g. navis.read_nrrd) and ret (navis.cut_skeleton) to determine the return value
• filenames/filepaths can be f or filepath
• MeshNeurons are constructed from (vertices, faces) tuple whereas Volumes take vertices, faces
• many functions use x as first positional argument regardless of whether that can be skeletons, meshes, dotprops or a combination thereof