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.