Decompose Element Natural Abundance

[blakec4]

When defining a material in MCNP, it is possible to provide the zaid for an element and not for the specific isotope.

 M6
     6000   -1.24E-4
     7014   -0.7552679999999999
     8016   -0.23178100000000001
     18000   -0.012827 $ Air 

This material definition would contain naturally occurring carbon (6000). When running MCNP, the element would be broken into its corresponding isotopes using the data that is in the xs_dir file for its natural abundances. In the current iteration, mcnpy just uses 'C0' for the material definition. In other Monte Carlo codes, such as OpenMC, there is no natural abundances for common elements in the data files. Instead, they are decomposed from the Python script for the final model file into the corresponding isotopes. When translating a MCNP input file into an OpenMC model, OpenMC will not know how to handle the 'C0' material as it does not have data. Materials should be decomposed when translating a model to ensure maximum compatibility for other codes (every Monte Carlo code will have data associated with specific isotopes but might not have abundance data to know how to handle natural materials).