Added __fspath__ method to Veg.
Added __fspath__ method to Veg.

Python 3.6 added support for __fspath__, which allows for marking
objects with path-like behavior to work as paths for functions and
methods in the standard lib. Eventually this should translate into all
datreant objects being treated like paths, but that is dependent on
downstream packages using e.g. `os.path.relpath` on their file path
inputs.

datreant/datreant,datreant/datreant,dotsdl/datreant,datreant/datreant.core,datreant/datreant.core