Add slot '.persistOnce' to class MxComputeSequence.
Add slot '.persistOnce' to class MxComputeSequence.

This will make custom compute plans easier for the user to work with.

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