ZZAAA form to PyNE's ID form (ZZZAAASSSS)

This adds the ability to represent metastable nuclides by adopting PyNE's nuclide ID format.  This also closes a potential ambiguity where elements (92000) were allowed to be members of compositions - with the exception of hydrogen which had to be a nuclide (1001).  Now only actual nuclides are allowed.

We should consider allowing users to specify nuclide string names instead of ids in the XML input file.

This closes #613