Support python function pipelines

[lukemarsden]

Enable a data scientist to define pachyderm pipeline steps as python functions (e.g. from a jupyter notebook).

[msteffen]

To add a bit of color:

• I think a python library could use getsource() to get the body of a python function/object, and then commit that to Pachyderm and run it in a standard python container. I believe kubeflow does this for function-based components.
• Deps could be downloaded and committed to pachyderm as well)