Fix issue 196.

If PETSc is already initialised (e.g. from importing petsc4py),
insert the command-line arguments via PetscOptionsInsert.
Fix GenericTensor/Vector::init hiding and cover with cpp unit test.
Move the PetscOptionsInsert call into the init_petsc
routine in SubSystemsManager.
Merge branch 'pefarrell/petsc-args-plus-petsc4py'
Merge branch 'garth/fix-vector-init-hiding'