Convert number density
\(\rho \left(\frac{\text{#}}{\text{nm}^3}\right)\)
into mass density
\(\rho_{\text{m}} \left(\frac{\text{kg}}{\text{m}^3}\right)\)
from the dictonary from function pa.density.bins() with
Dictionary returned from the pa.density.bins() Function
Mass density over the simulation box