Make `PetscTools::IsolateProcesses` affect `AmTopMost`, since that's now being used in the cell-based code.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18062 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2130 Get matrix pre-allocation correct for PETSc 3.3

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18065 b5f69653-2bf5-0310-8201-b0dafb8e1405