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 does the input (src)  has to be reset to zero here. Reset it if needed separately.
Does it actually ever happen that the src would have zero values?
The whole method appears to be not necessary at all: use a copy and possibly add a "reset" method to zero out all values (not sure why that would really be necessary)