prototype `python_pachyderm.mount()`

[albcui]

Mount a PFS repo locally, and be able to read/write data via the file system.

[albcui]

Depends on #318

[msteffen]

Our next goal for this issue is to get <User> to use in this lieu of manually-created data snapshots that their data engineers give to data scientists doing research. Sending snapshots to datascientists for development is a practice they seem to use, which seems onerous, and which this will hopefully automate