NPP expression

In the model of Mauritz the time dependent permeability due to NPPs was described by:

P = P_max * (1 + exp[(t_12 - t) / s])^-1 where P_max is the maximal permeability for an ion, t_12 is the timw of half-maximal permeability and s is the steepness of the permeability change.

Here we use this function to describe the expression of NPPs. We describe it with the time derivative as we use ODEs:

dNPP/dt = NPP_max * exp((t_12 - t) / s) / (s * (1 + exp((t_12 - t) / s))**2)