Add relativistic kinetic

[ErikQQY]

Add the relativistic kinetic from "Relativistic Monte Carlo" which is implemented in the research directory but has not been merged into the package.

There are still some TODOs:

• The usage of AdaptiveRejectionSampling.jl might have to be a hard dep, not sure how we can make it a weak dep here since we need them when we are making a random momentum for the phasepoint.
• Need some documentation and docstrings.
• Oh, it occurs to me that maybe we also need to first move these files from the research directory to the package repo so that the git history can be preserved?

[github-actions]

AdvancedHMC.jl documentation for PR #445 is available at:
https://TuringLang.github.io/AdvancedHMC.jl/previews/PR445/