account for possible different charges of ion populations

[nicolasaunai]

We can have multiple populations, each with its charge.
But the density used in Ohm and in Electrons is the total ion particledensity, which is wrong if populations have different charges.
We should allow for calculating the total charge density $\rho_c = \sum_{pop} Z_{pop}n_{pop}$

We should change rho_ to be the charge density, since the total particle density will not be used anyway.

[PhilipDeegan]

I think #861 closes this