Speedup Hit-in-Track, HitPattern and MultiTrackSelector

Modified the storage of hit-reference in a track into a simple pair of integers (and a RefCore)
fully backward compatible interface, including iorule.
Speedup MultiTrackSelector removing all overheads not due to MVA.
Part of the improvements are in HitPattern (more can be done, volunteers welcome)

Purely technical mods.
no regression expected.
no regression observed.
missed that pow...
and I agree looks like a bug (one of the many?)
never noticed because never executed
https://cmssdt.cern.ch/SDT/lxr/search?v=CMSSW_7_2_0_pre6&_filestring=&_string=useVtxError