Add CFPosition to pixelDigSimLinks; use in hitAssociator
Make associateHitbySimTrack a tracked param via existsAs
Fix merge conflicts from clash with #4714

Conflicts:
	SimTracker/SiPixelDigitizer/plugins/SiPixelDigitizer.cc
Copied over @wmtford's pixel digitiser changes to the Phase2 digitiser
Merge pull request #4738 from mark-grimes/resolve4716Conflicts

#4716 with merge conflicts resolved and Phase2 digitiser updated