DIPY unringing no longer works parallely

[tashrifbillah]

Our environment has dipy 1.1.0. The current version is 1.6.0. The current version's unringing program does not work parallelly:

pnlNipype/scripts/unring.py

Lines 52 to 65 in 9e805a2

	try:
	N_CPU= int(sys.argv[3])
	except:
	N_CPU= 4
	pool= Pool(N_CPU)
	signal.signal(signal.SIGINT, sigint_handler)
	try:
	pool.map_async(_unring, volumes)
	except KeyboardInterrupt:
	pool.terminate()
	else:
	pool.close()
	pool.join()

If we make it a for loop, the program works fine. But for loop is not an option of choice as it would take a very long time to unring 108 volumes (HCP-EP) data.

cc @yrathi @ShreyasFadnavis