Add `fps.pfsConfig=$pfsDesignId,$visit0,$convergenceDone` key.

We need to track when the PFI design and config change, and the fpsActor is the best placed to do that. At the start of a real convergence (`fps moveToPfsDesign designId=$pfsDesignId`), generate with `convergence="inProgress"`. When convergence is finished, regenerate with `convergence="done"`.
 
When running non-convergence moves (motormaps, say), invalidate the pfsDesign field.