Merge branch 'barry/2024-09-02/rebased-fix-conversion-warnings' into 'main'

Compiler finds (and forbid) casts from higher precision integers to lower

See merge request petsc/petsc!7806

Fix MPIU_* routines to always return MPI error codes and thus be usable with PetscCallMPI()

Introduce MPIU_Count to be used when MPI_Count is not available