Feature: Purge queues with a process group

[Chaffelson]

From this StackOverflow question: https://stackoverflow.com/q/48710410/4717963
It would be useful to have convenience methods to purge all the flowfiles from a PG, particularly when conducting testing or if you wanted to force delete the PG.

[Chaffelson]

Implemented.
canvas.get_connections
canvas.purge_connections
canvas.purge_process_group

example:
pg = nipyapi.canvas.get_process_group('myProcessGroup')
nipyapi.canvas.purge_process_group(pg, stop=True)