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)
Automatically ported from CMSSW_7_2_X #5136
why are these non-const methods here?
Who needs this interface? I see that only the ClusterSummary itself is using them. Why not just access the vectors directly?