Use direction of travel when looking up weight window mesh bin

[GuySten]

Description

When a geometry surface coincides with a plane of the weight window mesh, the weight window lookup at a surface checkpoint is done with the particle sitting exactly on that plane. The structured mesh index lookups (lower_bound_index for rectilinear meshes, ceil for regular meshes) resolve a position exactly on an element boundary to the element on the lower-coordinate side. As a result, a particle crossing such a surface in the +x/+y/+z direction is assigned the window of the element it is leaving, while a particle crossing in the opposite direction happens to get the correct one.

This was reported on Discourse (OpenMC's gamma transport with WW just slow or am I doing something wrong?) for a Cs-137 source in a W/Pb cask using ADVANTG weight windows whose mesh planes match the geometry. It showed up as an anisotropic relative error distribution and a much lower FOM than MCNP with the same weight windows. Another forum user traced the asymmetry to the weight-window lookup at surface checkpoints.

This PR offsets the lookup position by TINY_BIT along the particle's direction of travel in WeightWindows::get_weight_window, so a particle on a mesh boundary is assigned to the element it is entering. This is the same approach already used for tracklength mesh tallies in StructuredMesh::bins_crossed. Away from mesh boundaries the 1e-8 cm offset has no effect.

On a symmetric multigroup shielding problem with geometry planes on mesh planes and surface-only checkpoints, the relative error on the +x/+y/+z faces dropped from about 0.025 to about 0.016, matching the -x/-y/-z faces within statistics.

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)