Handle multigroup void velocity

[GuySten]

Description

Currently, void regions in multigroup mode makes openmc crash (#3723).

This PR implements the following changes:

1. Add warnings when mg data does not contain inverse-velocity.
2. When "void" inverse-velocity data is available use it to calculate particle speed in void regions.
3. When there is no data return particle speed of 0.

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)
• 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)

[paulromano]

@nelsonag would love to get your thoughts on this one!