change handling of const pointers in SysCopyHandle, const-retrieve in OverlapRemovalAlg

Essentially I now need to include the const modifier directly in the template type. The idea is that this allows future iterations of the handle to do different things based on whether they will provide read access or update access.

The change to OverlapRemovalAlg fixes a bug introduced in !55866 (merged), and is anyways what ought to be done.

PhysicsAnalysis/Algorithms/AsgAnalysisAlgorithms/AsgAnalysisAlgorithms/OverlapRemovalAlg.h

@@ -45,17 +45,17 @@ namespace CP
     SysListHandle m_systematicsList {this};
 
   private:
-    SysCopyHandle<xAOD::ElectronContainer> m_electronsHandle {
+    SysCopyHandle<const xAOD::ElectronContainer> m_electronsHandle {
       this, "electrons", "", "the electrons container to use"};
-    SysCopyHandle<xAOD::MuonContainer> m_muonsHandle {
+    SysCopyHandle<const xAOD::MuonContainer> m_muonsHandle {
       this, "muons", "", "the muons container to use"};
-    SysCopyHandle<xAOD::JetContainer> m_jetsHandle {
+    SysCopyHandle<const xAOD::JetContainer> m_jetsHandle {
       this, "jets", "", "the jets container to use"};
-    SysCopyHandle<xAOD::TauJetContainer> m_tausHandle {
+    SysCopyHandle<const xAOD::TauJetContainer> m_tausHandle {
       this, "taus", "", "the taus container to use"};
-    SysCopyHandle<xAOD::PhotonContainer> m_photonsHandle {
+    SysCopyHandle<const xAOD::PhotonContainer> m_photonsHandle {
       this, "photons", "", "the photons container to use"};
-    SysCopyHandle<xAOD::JetContainer> m_fatJetsHandle {
+    SysCopyHandle<const xAOD::JetContainer> m_fatJetsHandle {
       this, "fatJets", "", "the fat jets container to use"};
 
     /// \brief the decoration for the overlap removal status