Add chi-matrix support to type="super"

This PR continues to expand on superoperator representation support by adding ``superrep="chi"`` for chi (process) matrices in the Pauli basis. Currently, the unit tests fail on my system due to me misunderstanding how ``dims`` and ``tensor`` work together for ``type="super"`` ``Qobj``s (see line 92 of ``test_superop_reps.py`` and lines 161 and 173 of ``superop_reps.py``), but hopefully this should be a straightforward fix.

Once we have it working for qubits, we should be able to generalize to arbitrary dimensions by using the Heisenberg-Weyl group.