Allow File and Directory traits to accept py.path and pathlib objects

[effigies]

It's becoming increasingly common, particularly in Python 3, to use pathlib objects to describe files and directories. It would be useful to transparently accept these in File and Directory traits, converting them to strings.

It might also be useful (in Nipype 2.0) to add variants of these traits that are automatically promoted to pathlib objects when pulled from self.inputs.

Related: #2513