FIX-#425: Fix type of shared MPI.buffer

[Retribution98]

Mpi4py changed the default MPI.buffer type from signed char ('b') to unsigned char ('B'), which is the cause of incorrect data deserialization.

What do these changes do?

• first commit message and PR title follow format outlined here

  NOTE: If you edit the PR title to match this format, you need to add another commit (even if it's empty) or amend your last commit for the CI job that checks the PR title to pick up the new PR title.

• passes flake8 .
• passes black .
• signed commit with git commit -s
• Resolves [MPI] Fix hangs and errors when reading from shared memory #425
• tests added and passing
• module layout described at docs/developer/architecture.rst is up-to-date