BUG: basis elements not parsed correctly when tuple is used.

basis_elements = [['Zn', 'Sn', 'Cu'],
                  ['Se']]
basis = [(0., 0., 0.),         
         (0.5, 0.0, 0.75)]     

correctly leads to the second basis element as 'Se', but

basis_elements = [['Zn', 'Sn', 'Cu'],
                  ('Se')]
basis = [(0., 0., 0.),         
         (0.5, 0.0, 0.75)]   

leads to the second basis element as 'S' by only taking the first letter.