fixed implementation of mFactor in ConservationEquation

it now works properly with moist air

This is for #131 
It depends a bit on what you want to simulate of course. In my case I want to use mFactor to attribute more thermal mass to an air volume (zone model).

Consider the case where latent heat (both moisture and heat of evaporation) is added to the zone. The mass fraction Xi should then increase but the temperature of the zone should remain the same since latent heat is provided as well. This is not the case with the current implementation:
Due to the changed moisture level the specific internal energy ```medium.u``` changes as well. If the water were evaporated adiabatically this would cause the temperature of the zone to drop. In this illustration the heat of evaporation is added as well (to ```U```) so this should counteract the temperature drop. However due to the ```mFactor``` the mass is amplified and due to this not enough heat is supplied so the temperature does drop. This is non-physical and I would like to correct for this.

If this is not clear I can post an example.