Added specialized similarity transform

The similarity transform used by the track fit, and implemented in LHCbMath, is slightly too general for the matrices we are dealing with. This adds a (simple) explicit version which uses the symmetry in the transport matrices. It doesn't have any vectorised versions, yet. Should go together with Rec!2214 (merged)

Kernel/LHCbMath/LHCbMath/Similarity.h

@@ -64,6 +64,7 @@ namespace LHCb::Math {
 
     void similarity_5_1( span<const double, 15> Ci, span<const double, 5> Fi, span<double, 1> Ti );
     void similarity_5_5( span<const double, 15> Ci, span<const double, 25> Fi, span<double, 15> Ti );
+    void similarity_5_5_transport_matrix( span<const double, 15> Ci, span<const double, 25> Fi, span<double, 15> Ti );
     void similarity_5_7( span<const double, 15> Ci, span<const double, 35> Fi, span<double, 28> Ti );
     void similarity_2_5( span<const double, 3> Ci, span<const double, 10> Fi, span<double, 15> Ti );
 
@@ -96,6 +97,15 @@ namespace LHCb::Math {
     detail::similarity_5_5( detail::to_span( origin ), detail::to_span( F ), detail::to_span( target ) );
   }
 
+  // 'target' and 'origin' are NOT allowed to be the same object.
+  // specialized version of standard 5x5 similarity transform
+  // for track transport matrices. (i.e. for transporting covariances)
+  inline void Similarity_transport_matrix( const Gaudi::Matrix5x5& F, const Gaudi::SymMatrix5x5& origin,
+                                           Gaudi::SymMatrix5x5& target ) {
+    detail::similarity_5_5_transport_matrix( detail::to_span( origin ), detail::to_span( F ),
+                                             detail::to_span( target ) );
+  }
+
   inline Gaudi::SymMatrix5x5 Similarity( const Gaudi::Matrix5x5&    F, //
                                          const Gaudi::SymMatrix5x5& origin ) {
     Gaudi::SymMatrix5x5 target;