Added channelDensityNernst inside membraneProperties to Schema. Allows speciifcation
of channel densities where the rev potential is not fixed, but needs to be determined by
Nernst eqn & internal & external conc of ion flowing through
Improvements to handling of calculating current through nernst channel densities
Added fixedFactorConcentrationModel, where a fixed factor phi is used to scale the incoming
current INDEPENDENTLY OF THE SIZE OF THE COMPARTMENT to produce a concentration change.