KSC: new feature to add sixth order cross kerr terms

[qci-chou]

This implementation tries to follow what was done to add cross-kerr terms. If we are interested in adding generic coupling terms, maybe we want to do something more general. e.g. system.add_coupling_term('my_coupling_term', arbitrary_coupling_H0, strength)

[loganbvh]

If we are interested in adding generic coupling terms, maybe we want to do something more general. e.g. system.add_coupling_term('my_coupling_term', arbitrary_coupling_H0, strength)

I think I would prefer a general implementation rather a different one for each order, or putting this specific method on OCPSystem instead

[qci-chou]

I also agree that a general approach for adding coupling terms is the right thing to do. I can add this to OCPSystem instead.