Pre mixing modifications for 7 0 x

Modifications on top of 7_0_0 for premixing.  Changes include:

- full integration of a new DIGIPREMIX step into cmsDriver to get rid of customisation fragments
- use of label to pull proper payload out of the database to turn off Selective Readout in Ecal
- new sequence Digi_PreMix in Configuration/Standard sequences for digi options, to remove customisations
- new EventContent PREMIXRAW, again to remove customisation fragments

I will update the Twiki documentation for the PreMixing recipes.