Improved comments
Refactoring to address issue #8: multiple fluxes
Further to #8, test showing two fluxes in one compartment
Fixed #8: Added UnchargedSpecies class and don't set membrane_flux

This is to be applied to species who have had their net current
cancelled out int he mod file. The new tests test_twoMembraneSpecies
and test_twoMembraneSpeciesOneUncharged demonstrate this.
Updated to use UnchargedSpecies