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!

[nelsonag]

Why not tally a 1/v based on the group midpoint energy (I assume logarithmic midpoint is best) if no specific data is present? This could be done void or not.
I agree having data is better than not, but can assume this would work great for a UFG collapse sort of approach.

[GuySten]

Do you mean instead of defaulting to speed 0 when 1/v data is missing using velocity that match the logarithmic energy midpoint of the group?

[nelsonag]

Yep!

…

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