random ray source update loop

[shimwell]

Description

Precompute material array offsets outside inner loop in update_single_neutron_source().

Before: material * negroups_ computed repeatedly per group iteration
After: Computed once before loops as material_offset and scatter_offset

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)