--no_reg_template flag for images already in space-CITI168

[jordandekraker]

I was writing some additional tutorials/examples and came across a case where I think this sort of flag would be appropriate:
https://github.com/khanlab/hippunfold/blob/tutorials/docs/tutorials/specializedScans.rst#case-2-one-ex-vivo-hemisphere

Somebody actually emailed me with this exact case and is going to try proceeding with the --rigid_reg_template flag, but it may fail and I thought that adding an option that would simply use an identity matrix as the registration from space-T1w to space-corobl would be useful.

[jordandekraker]

merged and done