class AliITSVertexerCosmics: public AliITSVertexer

 This class implements a method to construct a "fake" primary
 vertex for cosmic events in which the muon crosses one of 5 inner
 ITS layers. A fake primary vertex is needed for the reconstruction,
 with e.g. AliITStrackerSA, of the two tracks produced by the muon
 in the ITS.
   We build pairs of clusters on a given layer and define the fake vertex as
 the mid-point of the straight line joining the two clusters.
   We use the innermost layer that has at least two clusters.
   We reject the background by requiring at least one cluster on the outer
 layer, closer than fMaxDistOnOuterLayer to the tracklet prolongation.
   We can reject (potentially pathological) events with the muon track
 tangential to the layer by the requiring the radial position of
 the vertex to be smaller than fMaxVtxRadius.
   Due to background clusters, more than one vertex per event can
 be found. We consider the first found.
   The errors on x,y,z of the vertex are calculated as errors on the mean
 of clusters coordinates. Non-diag elements of vertex cov. mat. are set to 0.
   The number of contributors set in the AliESDVertex object is the
 number of the layer on which the tracklet was built; if this number is -1,
 the procedure could not find a vertex position and by default
 the vertex coordinates are set to (0,0,0) with large errors (100,100,100)

 Origin: A.Dainese, andrea.dainese@lnl.infn.it