Broader support for SuperOperator type

[ytdHuang]

Support operator-bra and operator-ket like the ones in QuTiP.

Note that we wish to remain the dims::Vector{Int} to store only the Hilbert space dimension.
That is, if we have H = tensor(destroy(2), destroy(3)). Part of the output information of it's Liouvillian should be

julia > liouvillian(H)
Quantum Object:   type=SuperOperator   dims=[2, 3]   size=(36, 36)

and so does OperatorBraQuantumObject and OperatorKetQuantumObject