Add test that modifying Dat changes Vec norm
Ensure Vec context manager bumps object state

The PETSc Vec has a state counter and caches norm computations, since
the Vec is created pointing out our data, we need to bump this state
counter when returning the Vec context so that norms are recomputed.
Merge pull request #412 from OP2/vec-state

* vec-state:
  Ensure Vec context manager bumps object state
  Add test that modifying Dat changes Vec norm