Use mc high ka corr in time step

[ffoucart]

Proposed changes

Replace standard rate calculation with methods accounting for implicit Monte Carlo.

Upgrade instructions

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Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

[ffoucart]

Currently, this PR uses the time step instead of the light crossing time in the implicit MC method, and hard codes some free parameters... fixing the first part requires some thinking about GZ, that I'll get back to before merging.

[ffoucart]

Rebased, and added the cell light crossing time as an input to the TakeTimeStep function, per discussion at the Spectre technical code meeting.

[ffoucart]

Rebased against develop, updated comments to reflect latest changes. Should be ready for review now.