DNP drift (regular mesh only)

[JoffreyDorville]

Description

This PR introduces a new way to account for delayed neutron precursors (DNPs) drift in liquid-fueled systems. With this implementation, DNPs are transported using streamline integration (Runge Kutta 4). The velocity field is currently provided in the OpenMC model, but the end goal is to obtain it from Cardinal. This new feature is only available with regular meshes for the moment and will be extended to cover unstructured meshes in following PRs.

Because this PR uses the concept of field introduced in PR #3734, it is based on the corresponding branch.

This is the first step toward integrating the functionalities developed for the dnp_drift library presented in [1] directly into OpenMC source code.

This work is still in progress (draft PR).

References

[1] Joffrey Dorville and Paul K. Romano. “Explicit Transport of Delayed Neutron Precursors for Liquid-fueled Systems using OpenMC”, PHYSOR 2026, 19-23 April 2026, Turin, Italy (2026).

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)