modify xsreader to skip 1st line of xs file
Implement Material::setDensity(..); update addIsotope(..);

Material::setDensity(density, unit) only takes density in g/cc.
Material::addIsotope(isotope, atomic ratio) currently computes
isotope's number density by multiplying isotope's atomic ratio
and material's density. This is not correct -- I need the total
material atomic mass to compute real number densities.
computes isotopes' number densities correctly.

Material::addIsotope(.) function keep track of the isotops' atomic
ratio in this material and accumulate the total atomic mass for
this material (can add up to any number to be general), and when
we are done defining all the isotopes for the material, Material::
complete is called to update each isotope's number density by
the following formula:
isotope-number-den = rho-material * N_av * isotope-atomic-mass /
material-total-atomic-mass.