Merge branch 'do-not-default-construct-sourcelink' into 'master'

Do not default construct SourceLink

See merge request !641

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