Move to operator arithmetics

[KetpuntoG]

We should try and move our demos to using operator arithemetic (qml.dot,qml.sum, qml.prod ) instead of Tensor and Hamiltonian.

Tom: "This might be interesting to have as a PR and see what is failing. In terms of directing users to the new interface, we'd need to be sure that we've minimized the rough edges first."