Flag merged clusters for relaxed CPE errors

Add a function "refineCluster" in SiStripClusterizer to mark SiStrip clusters as merged. Use this information StripCPEfromTrackAngle to fall back to the larger "legacy" CPE errors for these clusters. The flag lives on the high bit of the firstStrip_ private data member of the SiStripCluster. Configurable thresholds in the sensor occupancy and strip width are used to discriminate between un- and merged candidates. The cluster refiner is turned off as configured for this PR. (Note that it's also overridden by useLegacyError = cms.bool(True) in StripCPEfromTrackingAngle_cfi.)

I'm closing PR #6348, which was based on 7_2_0_pre6, in favor of this one, based on 7_3_0_pre2, though I haven't tested execution in this release.