ENH: Add Physics-Based Non-Rigid Registration

Description:
This extension non-rigid registers a moving to a fixed MRI using a linear
homogeneous bio-mechanical model.

Contributors:
Fotis Drakopoulos (CRTC), Yixun Liu (CRTC), Andriy Kot (CRTC), Andrey Fedorov (SPL B&W Harvard),
Olivier Clatz (Asclepios INRIA), Nikos Chrisochoides (CRTC)
Center for Real-Time Computing (https://crtc.cs.odu.edu), Surgical Planning Lab,
Brigham and Women's Hospital (http://www.spl.harvard.edu/), Asclepios Inria (https://team.inria.fr/asclepios),
Old Dominion Univeristy and The College of William and Mary, Virginia, US