experimentally point at the mapdes petsc repos
Another attempt at our petsc branches
Fix PETSc install instructions

Change the install instructions, as well as the install script and the
requirements, to point to our branches of petsc and petsc4py.
Merge pull request #408 from OP2/updated_petsc_install

Fix PETSc install instructions
/petsc/ -> /mapdes/ for petsc4py instructions
compile for native machine arch