KalmanVertexUpdator performance tuning for use in AMVF

This MR significantly speeds up the KalmanVertexUpdator's update method by replacing a temporarily created and subsequently destroyed xAOD::Vertex object by a slim struct. This is also done in the IVertexUpdator and propagated to all clients. This, together with local caching of a matrix inversion result, yields a factor of up to 4 speed improvement for the Adaptive multi vertex fitter (see https://its.cern.ch/jira/browse/ATLIDTRKCP-245).

In addition to this performance tuning, some clean-up to the code is performed (removal of commented lines, attempt to clean up indentation to a consistent scheme, uniform use of messaging macros, use of std algorithms where possible, fix of potential memory leak).

The changes were tested to not change the reco output (except for CPU/timings) compared to running the AMVF in an unchanged master.

Adding @sroe and @npetters who are also working on improving the AMVF performance.

Tracking/TrkVertexFitter/TrkVertexFitterUtils/TrkVertexFitterUtils/KalmanVertexUpdator.h

@@ -75,7 +75,23 @@ namespace Trk
   float vertexPositionChi2(const xAOD::Vertex& old_vtx, const xAOD::Vertex& new_vtx) const;
 
   private:
-  
+  // struct used as return value for a faster version of positionUpdate 
+  struct positionUpdateOutcome{
+    Amg::Vector3D position; 
+    AmgSymMatrix(3) covariancePosition; 
+  }; 
+
+  // this method avoids the xAOD::Vertex return value of the public version, speeding up execution significantly. 
+  positionUpdateOutcome positionUpdateFast (const xAOD::Vertex& vtx, const LinearizedTrack * trk, double trackWeight, int sign) const;
+
+  // interface to the positionUpdateOutcome struct for internal use within positionUpdateFast
+  float trackParametersChi2(const positionUpdateOutcome& new_vtx, const LinearizedTrack * trk) const;
+  float vertexPositionChi2(const xAOD::Vertex& old_vtx, const positionUpdateOutcome& new_vtx) const;
+
+  // internal methods performing the actual calculations
+  float trackParametersChi2(const Amg::Vector3D & new_position, const LinearizedTrack * trk) const;
+  float vertexPositionChi2(const xAOD::Vertex& old_vtx, const Amg::Vector3D & new_position) const;
+
 /**
  * Method where the fit is actually done
  */