Add command-line interface for phase unwrapping

[tsalo]

The unwrapped phase data could be useful for purposed outside of distortion correction (e.g., phase regression, improved T2* estimation), so I was thinking it could be cool to have the unwrapping steps in warpkit as a command-line interface. I'm happy to open a PR if that sounds alright.

[vanandrew]

That sounds good. I sort of have that written at the moment with the --debug flag. But probably needs to be reworked into a proper save function.

[vanandrew]

Relevant lines for unwrapped phases when running with the --debug flag on:

warpkit/warpkit/unwrap.py

Lines 780 to 788 in f9d145f

	# Save out unwrapped phase for debugging
	if debug:
	logging.info("Saving unwrapped phase images...")
	for i_vol in range(unwrapped.shape[-2]):
	nib.Nifti1Image(
	unwrapped[:, :, :, i_vol, :],
	phase[0].affine,
	phase[0].header,
	).to_filename(f"phase{i_vol}.nii")