22.0-migration_gavrilenko-SiSPSeededTrackFinder

This is a migration of work undertaken by @gavrilen to attempt to speed up the SiSPSeededTrackFinder. Pinging also @ncalace. ATLIDTRKCP-250 Created to follow discussion and track supporting documents; discussion there indicates some ~20% speedup with little (0.13%, with pileup 40, 0.26% with pileup 60) efficiency loss. We will watch the SPOT plots, assuming this goes in, to track the CPU changes.