Improve memory management in ParticlePropertySvc

* replace `std::set<std::unique_ptr<ParticleProperty>>` with
  `std::deque<ParticleProperty>` to keep owned, stable (as
  in 'remains at the same memory location') properties

GaudiPartProp/src/ParticlePropertySvc.cpp

@@ -134,10 +134,8 @@ namespace Gaudi {
                                              double mass, double tlife, const std::string& evtName, int pythiaId,
                                              double maxWidth ) {
     //
-    auto i = m_owned.insert( std::make_unique<ParticleProperty>( particle, geantId, jetsetId, charge, mass, tlife,
-                                                                 evtName, pythiaId, maxWidth ) );
-    //
-    return i.second ? push_back( i.first->get() ) : StatusCode::FAILURE;
+    auto& pp = m_owned.emplace_back( particle, geantId, jetsetId, charge, mass, tlife, evtName, pythiaId, maxWidth );
+    return push_back( &pp );
   }
   // =============================================================================
   /// Add a particle property.

GaudiPartProp/src/ParticlePropertySvc.h

@@ -15,6 +15,7 @@
 // ============================================================================
 // STD&STL
 // ============================================================================
+#include <deque>
 #include <map>
 #include <set>
 // ============================================================================
@@ -180,8 +181,8 @@ namespace Gaudi {
     MapPythiaID m_pythiaidmap; ///< Map for Pythia Ids
 
     // local storage of ALL properties
-    std::set<std::unique_ptr<ParticleProperty>> m_owned;
-    std::set<std::string>                       m_replaced;
+    std::deque<ParticleProperty> m_owned;
+    std::set<std::string>        m_replaced;
 
     SmartIF<IFileAccess> m_fileAccess;
   };