Added OpenMC nuclide naming convention

I believe that the only difference between OpenMC and Serpent's names are for natural elements.  Serpent uses 'C-nat' while OpenMC uses 'C-Nat'.  Maybe @paulromano can confirm.
vals has to match the length of caseids, which is defined previously in the file.  None allows us to skip nuclides which are unrepresentable in a given format.