Preserve user material names in convert_to_multigroup

[shimwell]

Summary

convert_to_multigroup overwrote each material's name with a sanitised, HDF5-safe version, because the name was used both as the key for the material's XSdata entry in the MGXS library and as the macroscopic the material reads back. This silently changed user-assigned names, and two distinct materials whose names collided were written as a single cross section (one overwriting the other).

This keeps material.name untouched and instead keys the library by a separate name + id string (sanitised to alphanumeric/underscore; unique because the material ID is unique). The macroscopic reads its data under that library name. As a result:

• user material names are preserved (a material with no name stays unnamed),
• distinct materials that share a name no longer collapse — each gets its own cross section.

Verified end-to-end: a model with two distinct same-named materials converts and runs in multigroup mode, the library containing one entry per material.

Tests

Added a unit test asserting names are preserved and same-named materials map to distinct library entries.

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)