Datatype mismatch in parallel merger `mpimpi2prv`

While running MUST to troubleshoot the stuck merging job i also got an error from MUST that the parallel merger uses different datatypes on send on receive. (`MPI_INT` vs `MPI_UNSIGNED`)

<img width="1905" height="913" alt="Image" src="https://github.com/user-attachments/assets/520887a5-ed7a-4f82-b28a-97d9970746c3" />

See #141 for the whole MUST output with stacktraces :) 