Cluster summary rewrite

The ClusterSummary and ClusterSummaryProducer classes were updated to improve performance, as documented here:

https://its.cern.ch/jira/browse/CMSTRACK-51
(https://indico.cern.ch/event/322710/session/5/contribution/2/material/slides/1.pdf)
the detLabel has to match to what's encoded in detSelection enum value inside the ClusterSummary::CMSTracker.
To avoid problems keeping these in sync, in case a change is necessary, better use just one.
the string is a more readable choice (just don't do string manipulations every event ;) )