add preconditioning matrix to barker proposal

[ismael-mendoza]

Overview

This PR attempts to add a preconditioning matrix to the barker proposal implementation, as in appendix G of https://arxiv.org/abs/1908.11812

• Relevant discussion Barker proposal #723
• Relevant issue Implement mass matrix in Barker's kernel #725

Discussion / Questions

• Not sure if this is the best way to take the sqrt of the inverse mass matrix? I redo the computation of the inverse twice (tried to follow HMC but didn't seem appropriate to create a metric)
• Do I need to update the PDF? (not used anywhere in the codebase)
• I added step_size and the inverse_mass_matrix to the BarkerState (didnt' have any of these before), OK?
• It's not very clear in the Barker paper but the step_size (sigma) is a 'global scale' and the inverse mass matrix is separate. Are these redundant? See implementation from authors here (in R) - https://github.com/gzanella/barker/blob/master/functions.R

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[ismael-mendoza]

Closing because I jsut read the guidelines and noticed the commit and pre-commit requirements (will create a new PR)

[AdrienCorenflos]

Nw -> the commit pre-commit stuff is only last stage. In the meantime you can just try to get the logic right and then ask for a review when you think you have the Lego pieces fitting together.

[AdrienCorenflos]

Feel free to open a new pr or reopen this one, at the end of the day it doesn't matter much.

[ismael-mendoza]

Thanks @AdrienCorenflos that makes sense, I'll open a new one but remember that for next time. For reference opened #731