Do not default construct SourceLink

The KF previously default-constructed source link objects when inserting into the measurement map. Now it emplaces, therefore not placing the implicit requirement on source links to be default constructible (that's not explicitly required in the concept).

Core/include/Acts/Fitter/KalmanFitter.hpp

@@ -154,7 +154,7 @@ class KalmanFitter {
     std::map<const Surface*, source_link_t> inputMeasurements;
     for (const auto& sl : sourcelinks) {
       const Surface* srf = &sl.referenceSurface();
-      inputMeasurements[srf] = sl;  // copy!
+      inputMeasurements.emplace(srf, sl);
     }
 
     // Create the ActionList and AbortList