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
(even though it's /test)
don't go back to the configuration parameters PSEt every event, figure out the format and needed things at construction time. After that, the exception throw will be necessary only at construction