Split large MPI messages

Issues have been observed when MPI is used to send messages large enough that the count argument exceeds the range that can be represented by standard integers. However, no compiler warning was observed despite passing uint64_t for this argument. This should be investigated further and, eventually, a feature should be added to split large messages into smaller ones such that MPI can send them reliably.

Temporary abort-based check added in 9309df32.