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.