Add ICON, GradICON, and ConstrICON. Work in Progress

[HastingsGreer]

Description:

This pull request adds monai based implementations of the following papers:

Inverse Consistency by Construction for Multistep Deep Registration.
Hastings Greer, Lin Tian, Francois-Xavier Vialard, Roland Kwitt, Sylvain Bouix, Raul San Jose Estepar, Richard Rushmore, and Marc Niethammer
MICCAI 2023 https://arxiv.org/abs/2305.00087

ICON: Learning Regular Maps through Inverse Consistency.
Hastings Greer, Roland Kwitt, Francois-Xavier Vialard, Marc Niethammer.
ICCV 2021 https://arxiv.org/abs/2105.04459

GradICON: Approximate Diffeomorphisms via Gradient Inverse Consistency.
Lin Tian, Hastings Greer, Francois-Xavier Vialard, Roland Kwitt, Raúl San José Estépar, Marc Niethammer.
CVPR 2023 https://arxiv.org/abs/2206.05897

Not ready to merge:

One test is not passing

Pretrained weights for "Inverse Consistency by Construction for Multistep Deep Registration" are missing.

Types of changes:

Non-breaking change (fix or new feature that would not break existing functionality).

[HastingsGreer]

Next step: bring in uniGradICON pretrained weights