Required for #315 (further work needed)

Description

Exposes an efficient c++ and cuda implementation of the permutohedral lattice based filtering method discussed here...
https://graphics.stanford.edu/papers/permutohedral/

This was initialy implemented as an option for the back end of a bilateral filter. This PR is just exposing that functionality to allow other use cases.

Status

Ready

Types of changes

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