A generalized GN-model closed-form formula

The GN-model of fiber non-linearity has had quite substantial success in modern optical telecommunications networks as a design and management tool. A version of it, capable of handling arbitrary WDM combs and link structures in closed form, was proposed in 2014. Here we upgrade that formula, to make it capable of handling frequency-dependent dispersion, frequency-dependent loss and frequency-dependent gain/loss due to Stimulated Raman Scattering (SRS) among channels. This way, more challenging and complex network scenarios, like the ones that are being deployed right now, can be dealt with in real-time, to the great advantage of management, control and optimization of such networks.