Accept Python Path objects instead of only strings

[davidpagnon]

Python pathlib paths are much cleaner and easier to use than strings, and it is the recommended Python approach for manipulating paths since Python 3.6 (introduced in Python 3.4). When using OpenSim in other libraries that work with Path objects, it is a bit of a pain and a source of errors to have to convert the path into a string.

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Path does not do anything that os.path cannot do, it just makes it much easier and cleaner. Here are a few examples:

• your_path = Path(__file__).parent / 'models' / 'model.osim' vs
  your_path = os.path.join(os.path.dirname(os.path.abspath(__file__)), 'models', 'model.osim')
• your_path.stem vs
  os.path.splitext(os.path.basename(p))[0]
• your_path.with_suffix('.txt') vs
  good luck
• It also allows for more precise type hints: def process(path: Path) instead of def process(path: str)