fixed small bug in python interface of force.concentration_modulation (wrong lamellar direction)
fixed small bug in python interface of force.concentration_modulation (wrong lamellar direction)

refs #274

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue