#4716 with merge conflicts resolved and Phase2 digitiser updated

#4716 had some small clashes with #4714, fixed those and also copied over the changes to the Phase2 digitiser.  @wmtford could you look over my changes to SiPhase2Digitizer and SiPhase2DigitizerAlgorithm to make sure I've done it properly?  I pretty much just copied the changes you applied to SiPixelDigitizer and SiPixelDigitizer algorithm.