Fix SIGFPE (integer divide by zero) in MAPL_LoadBalanceMod

[mathomp4]

Summary

• Added safety check in MAPL_BalanceWork4 and MAPL_BalanceWork8 to handle Idim == 0 preventing division by zero.
• This occurs when the max work difference evaluates to 0 in load balancing, causing NumMax (and thus Idim) to be 0 (e.g. single sunlit gridpoint among processes).
• Updated CHANGELOG.md with an entry for this fix.

[mathomp4]

CC'ing @wmputman has he saw the issue this might fix

[mathomp4]

Per @wmputman this seems to have ... not hurt. That is, when he added this change, he did not get an FPE in MAPL anymore. The error just sort of "moved".

So my guess is this was acting more like a trap. Something bad happened and the divide-by-zero here was the first place the model tripped.

[mathomp4]

Ahh. I forgot about doing that. I need to remind myself!