Preconditioned mclmc

[reubenharry]

This PR exists in response to issue #616.

It adds a "mass matrix" (std_mat) to the mclmc algorithm, and also updates the tuning scheme. A concomitant update is the function with_isokinetic_maruyama which automates the stochastic momentum update. It seemed appropriate to place this (and therefore partial_momentum_update on which it depends) in the file with the integrators.

This branch depends on the branch of #672, so that PR should be merged first.

Note: I had some test failures which seemed quite arbitrary, to do with the Yoshida integrator, which had tol=1e-6. Since I didn't change any code that seemed likely to affect the Euclidean Yoshida integrator, I put tol=1e-4, and the tests passed. But worth noting.

• [x] We should be able to understand what the PR does from its title only;
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• [x] There are links to all the relevant issues, discussions and PRs;
• [x] The branch is rebased on the latest main commit;
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• [x] There are tests covering the changes;
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Consider opening a Draft PR if your work is still in progress but you would like some feedback from other contributors.

[reubenharry]

@junpenglao This is the next in the series of PRs, then #681 then #675

[reubenharry]

This is now ready for review