Class Coupling name

The class Coupling from the name seems very general and seems meant to be suited for any system. Yet I would argue that it is implicitly assuming that the systems are of type Transmon, Qubit, Resonator. For instance, already if you create a system of type Fluxonium (not yet there) having the option is_longitudinal does not make much sense physically and also the RWA is risky.

One option is to rename it as StandardCoupling or something like that and write in the description that they are standard for Qubit, Resonator, Transmon.

As an additional comment, I think handling of RWA is very tricky and users would like to handle it by themselves.