Support pathlib paths throughout pygeoprocessing

[phargogh]

This is one of those things where python's dynamic typing creates a little more work for us as maintainers.

Since python 3.4, the standard library has included pathlib, which provides an object hierarchy representing paths on various operating systems. GDAL requires a string, and if a pathlib.Path object is provided, an exception is raised. Fortunately, pygeoprocessing's docstrings are pretty clear that a str is required so this isn't a bug.

The workaround right now is to simply cast your path to a string before you call pygeoprocessing functions.