class AliTPCTracklet: public TObject

 The default constructor. It is intended to be used for I/O only.

 Contructor for a tracklet out of a track. Only clusters within a given
 sector are used.

TODO: write it!

 The copy constructor. You can copy tracklets!

 The assignment constructor. You can assign tracklets!

 The destructor. Yes, you can even destruct tracklets.

 Fit using Kalman filter

 y(x)=a[0]+a[1]*x+a[2]*x^2
 z(x)=b[0]+b[1]*x
 parametrises the corosponding helix at x0
 get the polynoms at x0

TODO: signs!

 The tracklet factory: It creates several tracklets out of a track. They
 are created for sectors that fullfill the constraint of having enough
 clusters inside. Futhermore you can specify the maximum amount of
 tracklets that are to be created.
 The tracklets appear in a sorted fashion, beginning with those having the
 most clusters.

 TODO!

 This function propagates two Tracklets to a common x-coordinate. This
 x is dermined as the one that is in the middle of the two tracklets (they
 are assumed to live on two distinct x-intervalls).
 The inner parametrisation of the outer Tracklet and the outer
 parametrisation of the inner Tracklet are used and propagated to this
 common x. This result is saved not inside the Tracklets but two new
 ExternalTrackParams are created (that means you might want to delete
 them).
 In the case that the alpha angles of the Tracklets differ both angles
 are tried out for this propagation.
 In case of any failure kFALSE is returned, no AliExternalTrackParam
 is created und the pointers are set to 0.

 This function generates a n-dimensional random variable x with mean
 p and covariance c.
 That is done using the cholesky decomposition of the covariance matrix,
 , with  being an
 upper triangular matrix. Given a vector v of iid gausian random variables
 with variance 1 one obtains the asked result as:
.
 c is expected to be in a lower triangular format:
 c[0]
 c[1] c[2]
 c[3] c[4] c[5]
 etc.

    aliroot
    AliTPCTracklet::Test("");
    TFile f("AliTPCTrackletDebug.root");
    TTree *t=f.Get("AliTPCTrackletDebug");
    t->Draw("p0:p4");
    TEllipse e=AliTPCTracklet::ErrorEllipse(0.,0.,4.,1.,1.8);
    e.Draw();

 check the acceptance of cluster
 Cut on edge effects

 Returns the tracklet parametrisation at its outer most cluster.

 Returns the tracklet parametrisation at its inner most cluster.

 Returns the tracklet parametrisation at X=0, i.e. the "primary vertex".

 Returns the sector in which the tracklet lives.

 Returns the number of clusters assined to the tracklet.

 Returns the clusters of this tracklet. In case they weren't stored it
 returns 0.