Fix x-axis related error treatments

Inspired by the refining (@galocco) of similar errors in the na60+ adaptation of DCAFitter (@fprino).

List of changes:
1. TrackCovI now stores a full symmetric 3D information matrix.
   The old dummy X variance is removed.  The X information is derived from
   the track slopes dY/dX and dZ/dX via H^T C_YZ^{-1} H.

2. XerrFactor was removed.
   The fit no longer creates an artificial longitudinal uncertainty from
   sigma_Y.

3. getTrackCovMatrix() was removed.
   PCA covariance is now computed from summed track information matrices,
   not by rotating an artificial 3D covariance with dummy X error.

4. calcPCACoefs() now multiplies the full local information matrix.
   Terms involving local XY and XZ correlations are included in the Ti
   coefficient matrices.

5. calcInverseWeight() now rotates and sums the full information matrix.
   All six local information elements contribute to the global 3x3 weight.

6. calcChi2Derivatives() now contracts residual derivatives with the full
   information matrix.
   The Hessian curvature term is added only to diagonal Hessian elements,
   matching the fact that each trajectory has second derivative only with
   respect to its own running parameter.

7. calcChi2DerivativesNoErr() was corrected analogously.
   The Gauss-Newton term contributes to all Hessian elements, while the
   residual-times-second-derivative term contributes only to diagonal
   elements.

8. calcPCACovMatrix() now returns the inverse of summed track information.
   The longitudinal vertex uncertainty comes from track slopes rather than
   from a dummy X covariance.

9. calcChi2() now uses all six information-matrix elements.
   Residual correlations involving X are included in the chi2 value.

10. correctTracks() now propagates mCandTr analytically to the corrected X.
    This keeps the candidate track state, slopes, covariance, and mTrPos
    synchronized between Newton iterations while avoiding full-field
    Propagator calls for the small Newton corrections.

11. createParentTrackParCov() now builds the parent momentum covariance from
    the native O2 momentum parameters (snp,tgl,q/pt).
    The Jacobian includes the track-frame alpha rotation, and daughter
    momentum covariances are summed in lab px,py,pz before the existing O2
    parent TrackParCov constructor converts to the parent frame.

Author: shahor02