small change to facilitate massless particles

(note the opencl kernel should be tested for performance loss)
Added legacy functions set_kz(), get_kz(), set_eta(), get_eta(), set_etai(), get_etai(), set_etar(), set_etar(), set_rbar(), get_rbar(), set_zmbar(), get_zmbar(); defined 3 new parameters: timestep_parameter, epsilon_squared, begin_time.
Added legacy functions set_kz(), get_kz(), set_eta(), get_eta(), set_etai(), get_etai(), set_etar(), set_etar(), set_rbar(), get_rbar(), set_zmbar(), get_zmbar(); defined 3 new parameters: timestep_parameter, epsilon_squared, begin_time.