Revert "Make slot '.persistOnce' more backwards-compatible."
Revert "Make slot '.persistOnce' more backwards-compatible."

This reverts commit e38dc9110dec820351e7cb629e2f53d3680dcf1e.

jpritikin/OpenMx,mhunter1/OpenMx,jpritikin/OpenMx,mhunter1/OpenMx,jpritikin/OpenMx,mhunter1/OpenMx,jpritikin/OpenMx,mhunter1/OpenMx,jpritikin/OpenMx,jpritikin/OpenMx,mhunter1/OpenMx,mhunter1/OpenMx