Allow tracklength estimator for neutron heating

[shimwell]

Description

When photon transport is enabled, heating tallies currently get switched to the collision estimator unconditionally.

Is it possible to keep neutron only tallies using tracklength estimator

I think this is also useful alongside #3771 to be able to tally things like heating in silicon from neutrons using the tracklength estimator.

I might be missing some edge cases here as this is not an area of the code I frequent so sorry if I'm way out here.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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)

[paulromano]

Makes sense; thanks @shimwell!