Add the ability to tally microscopic cross sections in void materials

[GuySten]

Description

This PR add the ability to tally microscopic cross sections in void materials.

Fixes #3412.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[lewisgross1296]

Just for my own curiosity, what is the application/use case of this?

[GuySten]

It lets you calculate reaction rates in void regions without touching the model.
For example it can be used for calculation of equivalent thermal neutron flux (equivalent with respect to some thermal microscopic cross section).

[shimwell]

Many thanks for this PR

I checked out the branch and made a small simulation obtaining dose on silicon and comparing to biological dose

Yes this does look useful as one does not need to change the goemetry to know what the dose on silicon would be in a location.

Two questions.

• Could / should this PR support photon tallies as well as neutron tallies or should that be a follow up PR?
• Should this PR produce tallies in void space, perhaps i have a mistake in my script but for me it didn't give tallies in void space (the biological dose did but not the silicon dose)

https://github.com/fusion-energy/neutronics-workshop/blob/adding-si-dose-overlap/tasks/task_09_CSG_instantaneous_dose_tallies/6_silicon_dose_on_mesh.py