Added new function AliL3Hough::MergeEtaSlices which merges tracks which

Added new function AliL3Hough::MergeEtaSlices which merges tracks which
appear in 2 neighbouring eta slices. The tracks are matching if they have
identical kappa and psi; they appear in the same bin. The one with the
lower threshold are removed from the track array.

There is also a bugfix in AliL3HoughTransformer::GetEta, the sign should
not be swicthed on the other half of TPC, because the sign is already included
in the (GetEtaMax() - GetEtaMin()).

Also the DoMC flag in the AliL3HoughTransformer ctor.