implement Isotope::getDistanceTraveled(neutron).

Nothing fancy -- distance travelled is -ln(random-number)/Total-
xs of the isotope.
Update Material::addIsotope(..)

Removed the Material::complete() function; instead of having to
explicitly states that a material is complete, the addIsotope(..)
function now updates all isotope's number densities every time
a new isotope is added by multiplying the old isotope number
density by the ratio of the new material atomic mass over old
material atomic mass.

One problem is that we might now require user to input atomic
ratios that add up to 1 for each material. I don't have a way to
go around it.
Merge branch 'master' of https://github.com/wbinventor/PINSPEC