petsc::mpi re-exports compatible MPI
This means examples don't need to deal with versioning of the mpi package and ensuring that a version consistent with libpetsc.so is found. We now recommend most examples use a statement like this.
use petsc::{prelude::*, mpi::traits::*};
One can now use petsc.world().size() without needing to think about MPI beyond the import above.
Fixes: #6 (closed)