support pathlib.Path for onto.save()

[JeffreyAnimal]

Add support for Path objects for saving Ontology

It would be great if pathlib.Path objects would be supported by the save() function.

Currently a Exception is thrown:

 AttributeError: 'PosixPath' object has no attribute 'write'

Current Code

save_path = pathlib.Path(~/example_path) / "save_file.nt"
onto.save(file=str(save_path), format="ntriples")

Expected Code

save_path = pathlib.Path(~/example_path) / "save_file.nt"
onto.save(file=save_path, format="ntriples")