Coupling sections assume harmonic matrix elements

[DanielSank]

The coupling sections assume that the system is approximately harmonic. We should make clear where that assumption is made and include formulae (for g) in terms of general matrix elements.

Relatedly, we need to make it very clear as to why Q_zpf and Phi_zpf use the "partial impedance", or if using the zero point quantities makes sense in the first place.