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/TrkVertexFitterInterfaces/TrkVertexFitterInterfaces/IVertexUpdator.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 ///////////////////////////////////////////////////////////////////
@@ -55,6 +55,21 @@ namespace Trk
         */
        static const InterfaceID& interfaceID() { return IID_IVertexUpdator; };
      
+       // Slim struct used to transport the (temporary) result of positionUpdate. 
+       // Significantly faster than using an xAOD::Vertex for this purpose. 
+       struct positionUpdateOutcome{
+          Amg::Vector3D position;       // vertex position
+          AmgSymMatrix(3) covariancePosition;   // position covariance
+          AmgSymMatrix(5) trkParametersWeight;  // inverse track parameter covariance, 
+                                                // cached to repeat re-calculation
+       }; 
+
+       // operating mode for positionUpdate. 
+       typedef enum{
+          removeTrack=-1, 
+          addTrack=1 
+       } updateMode;
+
       /**
        * Add method: adds one track to a vertex
        */
@@ -70,17 +85,21 @@ namespace Trk
       * Updates a position with the information from 
       * one track. 
       */
-       virtual xAOD::Vertex positionUpdate (const xAOD::Vertex& vtx, const LinearizedTrack * trk, double trackWeight, int sign) const = 0;
+       virtual positionUpdateOutcome positionUpdate (const xAOD::Vertex& vtx, const LinearizedTrack * trk, double trackWeight, updateMode mode) const = 0;
        
        /**
-       * Method calculating the interstep Chi2 increment
+       * Method calculating the interstep Chi2 increment.
+       * Can also be called using the outcome from a prior positionUpdate call. 
        */ 
        virtual float trackParametersChi2(const xAOD::Vertex& new_vtx, const LinearizedTrack * trk) const = 0;
+       virtual float trackParametersChi2(const positionUpdateOutcome & new_vtx , const LinearizedTrack * trk) const = 0;
 
        /** 
        * Method calculating the vertex displacement-related part of the chi2   
+       * Can also be called using the outcome from a prior positionUpdate call. 
        */
        virtual float vertexPositionChi2(const xAOD::Vertex& old_vtx, const xAOD::Vertex& new_vtx) const = 0;
+       virtual float vertexPositionChi2(const xAOD::Vertex& old_vtx, const positionUpdateOutcome & new_vtx) const = 0;
 
   };
 }