updated to fix material.mcnp issue

The material::mcnp method would produce valid mcnp formatted materials, however a given nuclide has an abundance of zero, the method would produce a correctly formatted entry for mcnp. Abundance zero nucids creep in when loading materials from a material library. However, mcnp does not like to have abundance 0 nuclides in the material, this modification only adds the nucid and composition abundance if the abundance is greater than 0, test is included to prove that this works. 
missed one here. http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Conditionals