Make main workflow callable from Python

[tsalo]

Summary

Currently, heudiconv's main workflow is included in the same function that parses the CLI arguments. With a little refactoring, we could separate these two a bit to allow users to call heudiconv from within Python. I think it would be fairly easy, and it would help users like me who prefer to call heudiconv within larger Python scripts.

This is somewhat related to this NeuroStars post (specifically point 2).

[mgxd]

FWIW, we hack around this in the tests like so:

heudiconv/heudiconv/tests/test_regression.py

Line 8 in a1a7cb5

from heudiconv.cli.run import main as runner

although definitely onboard with separating the workflow into its own function / script.