add get_opt_positions to get optimized structure for MOPAC optimization

The original demo in https://wiki.fysik.dtu.dk/ase/ase/calculators/mopac.html#mopac cannot get the new structure after optimization from Mopac.

Test pass:

atoms.calc = MOPAC(label='H2', task='GRADIENTS').

atoms.calc = MOPAC(label='H2', task='1SCF GRADIENTS').

1SCF will return same structure without error.

let me know if we prefer update positions into current atoms object.