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.
why not use powN here (considering that this piece is inside of a loop)?
.. the ```pow(nlayers, param[1])``` could even be reused from above and ```pow(param[0], param[1])``` done above computed as a const outside of all the loops, to reduce number of pows (``` pow(nlayers, param[1])``` and ```pow(param[0]*nlayers, param[1])```

In fact, the difference in use of param[0] here and above makes me think there is a bug in the code, otherwise why would one dz cut should be done with exponentiating the parameter and the other with the parameter outside of the pow