Make ParameterSet store covariance as optional instead of pointer

This should have the advantage of it being local to the class, rather than a heap allocation that the client has to perform and pass to ParameterSet.

Core/include/Acts/EventData/ParameterSet.hpp

@@ -310,7 +310,7 @@ class ParameterSet {
    *
    * @return raw pointer to covariance matrix (can be a nullptr)
    */
-  const std::optional<CovMatrix_t> getCovariance() const {
+  const std::optional<CovMatrix_t>& getCovariance() const {
     return m_optCovariance;
   }