Compute Atomic Number Density from Weight Percent

The PySCL ability to calculate number densities from a weight percent isotopic distribution is needed for PyARC.

The formula for this calculation is as follows : aden(i) = wtpt(i)*rho(mi)*avogadro/(mass(i)*1E+24) => aden(i) = wtpt(i)*rho(mi)/mass(i)*(avogadro/1E+24) for each isotope i in mixture mi.