Use correct density as initial condition for Picard iterations

[paulromano]

Moving issue 31 from gitlab originally submitted by @aprilnovak:

If we're going to save densities between Picard iterations and possibly use them for computing norms, we need to save the initial value used in the neutronics solver (since it runs first). So, CoupledDriver should add densities_ and densities_prev_ members, where densities_prev for the first iteration is either:

• read from the OpenMC input file, or

• computed based on Nek temperatures from a restart file using IAPWS

We can indicate which of these two options we would like to use by adding another XML tag in enrico.xml - density_ic, similar to temperature_ic.