Neutronics PKAGenerator & averaged species concentration

[snschune]

When neutronics results are used to compute the number of recoils, we need to take into account the two relevant length scales: cm for neutronics [both species' number densities and neutron fluxes vary on that length scale] and mu-m for BCMC/micro-scale where number densities change over the length scale of grains. Information entering the recoil generators are often multiplied by a macroscopic number density, aka the average of number density over the micro-domain. To still have the rasterized variables be number densities and not a fluctuation over the average number density [number density divided by the average of the number density over the micro-domain], we need to couple in a PP to the PKAGenerators where this is applicable. Also this should be properly documented.