Enable using pathlib.Path for file references

[JosephCottam]

Since python 3.4, pathlib.Path is the preferred high-level file-path work (and building up path strings is often considered high-level). This is in place of string-composition (even through os.path.join). Many current interfaces that may take a path (e.g., load_petri_model) test for string-or-object, but this cause a problem if a Path is passed.

The current work-around is to convert the path to a string before passing it in, but I propose that we update the allowed Union to include pathlib.Path objects and add that support to the corresponding functions.