Change defining Parameter interface for NeutralParticle to be the same as…

Change defining Parameter interface for NeutralParticle to be the same as TrackParticle, cleaner and MT compatible. i.e the static and returning by reference are now automatic and by value (like the trackParticle).

There is a case clients depend on this "feature" , hopefully this will not be the case i.e the CI will tell us

Event/xAOD/xAODTracking/Root/NeutralParticle_v1.cxx

@@ -109,8 +109,8 @@ namespace xAOD {
   AUXSTORE_PRIMITIVE_GETTER(NeutralParticle_v1, float, theta)
   AUXSTORE_PRIMITIVE_GETTER(NeutralParticle_v1, float, oneOverP)
 
-  const DefiningParameters_t& NeutralParticle_v1::definingParameters() const{
-      static DefiningParameters_t tmp;
+  const DefiningParameters_t NeutralParticle_v1::definingParameters() const{
+      DefiningParameters_t tmp;
       tmp << d0(),z0(),phi0(),theta(),oneOverP();
       return tmp;
   }
@@ -154,11 +154,12 @@ namespace xAOD {
             v.push_back(cov(icol,irow));
   }
 
-  const xAOD::ParametersCovMatrix_t& NeutralParticle_v1::definingParametersCovMatrix() const {
+  const xAOD::ParametersCovMatrix_t NeutralParticle_v1::definingParametersCovMatrix() const {
     static const Accessor< std::vector<float> > acc( "definingParametersCovMatrix" );
     std::vector<float> v = acc(*this);
     std::vector<float>::const_iterator it = v.begin();
-    static xAOD::ParametersCovMatrix_t cov; cov.setZero();
+    xAOD::ParametersCovMatrix_t cov; 
+    cov.setZero();
     for (size_t irow = 0; irow<5; ++irow)
         for (size_t icol =0; icol<=irow; ++icol)
             cov.fillSymmetric(icol,irow, *it++);