code: def dens_alum_nanocluster(coag):
      
      density = (coag.PrecipDensity * MOLEC_WEIGHT_ALUMINUM
                 * coag.PrecipAluminumMPM / coag.PrecipMolecWeight)
      return density
docs: Return the density of the aluminum in the nanocluster.
  
      This is useful for determining the volume of nanoclusters
      given a concentration of aluminum.