Beamspot nominal collision2015

- introduce beamspot parameters as agreed during PPD General discussion https://indico.cern.ch/event/341218/ , NominalCollision2015
- the defaualt beamspot (e.g. as used by ConfigBuilder if no option is specified) remains as before : Realistic8TeVCollision
- use the NominalCollision2015 for postLs1 relvals (explicit cmsDriver.py option added)