For #2341, add `PetscTools::GetWorld()`, an alternative to `PETSC_COMM_WORLD` that understands process isolation.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19059 b5f69653-2bf5-0310-8201-b0dafb8e1405
For #2341, use `PetscTools::GetWorld()` in cell-based Chaste, to fix the WISC project.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19060 b5f69653-2bf5-0310-8201-b0dafb8e1405