sbend using chef_mixed extractor_type with Independent_stepper_elements introduces spurious motion
In the attached bad_lattice.lat and run_bad_lattice.py, the particle starting at 0 becomes significantly non-zero when propagated through the sbend. This only happens with the Independent_stepper_elements stepper, not the Independent_stepper. CHEF propagating through the elements doesn't show any spurious motion.
I've changed the default extractor_type to chef_propagate, but we still use chef_mixed for special runs so this shouldn't be forgotten.