TrackBase & HitPattern redesign discussed on the RECO meeting of 08/05/2014

Documentation about the changes:
* https://twiki.cern.ch/twiki/bin/viewauth/CMS/TrackingHitPatternRedesign
* https://indico.cern.ch/event/327055/contribution/6/material/slides/0.pdf

it is quite misleading to call this method ::set, while it never resets (unlike the previous version).
add or append would make more sense.
Also, please add a resetHitPattern method.