Crash of PySCL when using natural abundance elements

@lefebvre I noticed PyARC/PySCL will crash when using natural abundance material. With MCC3's libraries, there are at least three elements that should be supported: "c", "zn" and "v". Could you please check this? I expect it should be an easy fix (I don't see why this does not work)... Here is the error message I get:

Traceback (most recent call last):
  File "/Users/nstauff/PyARC/PyARC.py", line 2245, in <module>
    this.execute(sys.argv[1:])
  File "/Users/nstauff/PyARC/PyARC.py", line 473, in execute
    self.prerun(options)
  File "/Users/nstauff/PyARC/PyARC.py", line 1140, in prerun
    self.load_mixing_table()
  File "/Users/nstauff/PyARC/PyARC.py", line 944, in load_mixing_table
    self.load_adens(self.mixing_table,material)
  File "/Users/nstauff/PyARC/PyARC.py", line 916, in load_adens
    mixture.add_atom_den(float(aden.value.data), self.sclib.nuclide(str(aden.id)))
  File "/Users/nstauff/PyARC/PySCL/PySCL.py", line 272, in add_atom_den
    self.atom_densities[inventory.identifier()] = atom_density
AttributeError: 'NoneType' object has no attribute 'identifier'

Process finished with 0 return code; ran in 4 secs, finished at Tue Jun 13 12:42:16 2017

Thanks!