Add support for other coupling operators

[hodgestar]

Currently the interaction operators supported are XX, YY, and ZZ. It would be nice to have support for more general coupling operators such as XZ.

[hodgestar]

One nice way to handle the whole operator thing would be to add the concept of a custom operator that we can use in a few different places (e.g. for expectation values).

If the user selects custom, they can then edit the operator in the details section and write their own. E.g. "XX" could be the operator "XX", but the user could also write "XZ" or "XX + YY" or similar.

[hodgestar]

Boxi suggested that we just add interaction options for "XY", "XZ", "YZ" and "YX", "ZX", "ZY" and that will cover all 9 possibilities.

[ajgpitch]

I think it should be fairly easy within the interface, that when making a coupling, to have radio buttons for qubit_i operator and qubit_j operator. This would add most of the flexibility that is suggested here.