Reference Space Warp Transformation for ZVZB Forces

[rcclay]

Proposed changes

This implements a preliminary reference version of the space-warp transformation for ZVZB force estimators, given in https://doi.org/10.1063/1.3516208. Currently, Grad_i(E_L) w.r.t. electron coordinates is handled through finite differences, but replacement with analytic derivatives should reasonably occur within the current infrastructure.

What type(s) of changes does this code introduce?

• New feature

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Local intel Xeon server.

Checklist

• Yes. This PR is up to date with current the current state of 'develop'
• No. Code added or changed in the PR has been clang-formatted
• Yes. This PR adds tests to cover any new code, or to catch a bug that is being fixed.
• No. Documentation has been added (if appropriate)

[ye-luo]

Please clang-format the modified source code will also be helpful.

[rcclay]

@ye-luo OK. Short of the making ions "const" issue and the default behavior of evaluateDeterministic, I believe I have addressed all your concerns.

[ye-luo]

Thank you for helping with the code quality improvements. Only a few minor issues left.