Fix get_index_in_direction for regular meshes

[JoffreyDorville]

Description

For regular meshes, the function get_index_in_direction is returning 0 instead of 1 when the tested coordinate is coincident with the lower boundary. This results in points being incorrectly seen outside the mesh when they lie on surfaces corresponding to the lower boundaries of the mesh.

For rectilinear meshes, the function get_index_in_direction works as expected.

I added a cpp unit test to illustrate the problem and to show that this PR fixes it.

This fix also corrected the results of the weightwindows/survival_biasing regression tests which have been updated. I cross-checked that using an equivalent rectilinear mesh instead of the regular mesh in tests/regression_tests/weightwindows/survival_biasing/test.py produces the same results as the updated ones with the fix.

Rectilinear mesh used:

mesh = openmc.RectilinearMesh()
mesh.x_grid = np.linspace(0.0, 160.0, 21)
mesh.y_grid = np.linspace(0.0, 160.0, 21)
mesh.z_grid = np.linspace(0.0, 160.0, 2)

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)

[GuySten]

Looks good to me.

[paulromano]

I'm not sure I agree with the change. When the position being tested is on the boundary between two mesh elements, it is ambiguous as to whether it should be considered as being in the mesh element to the left or to the right. To me it seems more consistent to always pick one direction.

We have other ways of handling this ambiguity elsewhere in the code such as 1) pushing the position forward by TINY_BIT to move it off the boundary, or 2) Looking at the dot product of u and the surface normal to decide which one to choose.

[GuySten]

I think using the particle direction is a good solution.
In any case, equivalent regular and rectilinear meshes should have the same behavior.
Currently they behave differently.