New pixel sim link

This PR does for the pixels what PR 4678 does for the strips:  enables (and uses in the hitAssociator) direct access to the simHits via the PixelDigiSimLinks.  (See my presentation at the July 11 Simulation meeting, https://indico.cern.ch/event/329213/.)  The change for the pixels is a little deeper than for the strips, since until now the pixel simLinks never supported simHit pointers, CFposition().  The size of a simlink thus grows from 16 to 20 byates.