Add Riemannian manifold HMC

[ErikQQY]

Add the Riemannian manifold HMC from "Riemann manifold Langevin and Hamiltonian Monte Carlo methods" which is implemented in the research directory but has not been merged into the package. Supersede #432

Though this PR is relatively complete, there are still some things TODO:

• Discuss the appropriate style for constructing a DenseRiemannianMetric, e.g., how should the metric tensor G be provided, etc.

• Still need to add some docstrings and related documentation.

[ErikQQY]

Does files changed in PR also indicate renaming? Seems commit 1377742 does have the renaming history.

[yebai]

@ErikQQY I renamed the files in #438 to make it easier for Git to track these changes. Can you create a new PR (or force push to this PR) with only the last commit 6d354da?

[ErikQQY]

@ErikQQY I renamed the files in #438 to make it easier for Git to track these changes. Can you create a new PR (or force push to this PR) with only the last commit 6d354da?

Sure! This PR get a bit messy, close in favor of #439