1405 add bending energy loss

[kate-sann5100]

Fixes #1408 .

Description

Add bending energy loss useful for registration.

Status

Ready/Work in progress/Hold

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh --codeformat --coverage.
• Quick tests passed locally by running ./runtests.sh --quick.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[wyli]

thanks, the style/format of this PR looks great, it's a really nice starting point. I put some comments about the actual logic. ideally we should have something general for different spatial dimensions, not just for spatial 3D

[wyli]

thanks, I forgot to mention that perhaps rename gradient to spatial_gradient to avoid confusion?

also, do you want to put some acknowledgement in the docstring such asadapted from DeepRegNet? cc @YipengHu

[YipengHu]

thanks, I forgot to mention that perhaps rename gradient to spatial_gradient to avoid confusion?

also, do you want to put some acknowledgement in the docstring such asadapted from DeepRegNet? cc @YipengHu

Sounds good to have a reference to DeepReg, using either the paper or github URL.
"spatial_gradient" is more specific and better.