* Possibility to reconstruct cosmic muon tracks that cross the ITS

* Possibility to reconstruct cosmic muon tracks that cross the ITS
at a distance from the origin smaller than the radius of the inner SSD layer:
expected to be useful for ITS alignment.
(before we reconstructed only muons crossing the inner SPD layer)
Modified classes:
-AliITSVertexerCosmics: it now builds the "fake" vertex using any layer (<SSD2)
with at least two clusters
-AliITStrackerSA: it now finds tracks with clusters on the N outer layers
(2<=N<=6). The default is still to require all 6 layers.
-AliITSReconstructor. In the case of "cosmics" setting, it enables
the above feature in AliITStrackerSA.

Andrea