added MLL_OSSF selection algo

Added MLL_OSSF algorithm that calculates the invariant mass of two OS electrons/muons and selects the events with the mass in user-defined range in event selection. Use keyword MLL_OSSF to call the algorithm in event selection,MLL_OSSF 80000 100000 selects events with at least 1 Z-like pair, whereas MLL_OSSF 80000 100000 veto vetoes them.

this is originally needed by 4tops analysis, to avoid checking out this package locally and hopefully could benefit other analysis as well, I'm moving it to athena in this MR.

Tagging @ravinab for approval.

PhysicsAnalysis/Algorithms/EventSelectionAlgorithms/EventSelectionAlgorithms/DileptonOSSFInvariantMassWindowSelectorAlg.h

+/*
+   Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
+*/
+
+/// @author Binbin Dong
+
+#ifndef EVENT_SELECTOR_DILEPTONOSSFINVARIANTMASSWINDOWSELECTORALG_H
+#define EVENT_SELECTOR_DILEPTONOSSFINVARIANTMASSWINDOWSELECTORALG_H
+
+// Algorithm includes
+#include <AnaAlgorithm/AnaAlgorithm.h>
+#include <AsgTools/PropertyWrapper.h>
+#include <SystematicsHandles/SysReadHandle.h>
+#include <SelectionHelpers/SysReadSelectionHandle.h>
+#include <SelectionHelpers/SysWriteSelectionHandle.h>
+
+// Framework includes
+#include <xAODEgamma/ElectronContainer.h>
+#include <xAODMuon/MuonContainer.h>
+#include <xAODEventInfo/EventInfo.h>
+
+namespace CP {
+  /// \brief an algorithm that vetos an event with two OS electrons or muons mass in a specific mass window
+
+  class DileptonOSSFInvariantMassWindowSelectorAlg final : public EL::AnaAlgorithm {
+
+    /// \brief the standard constructor
+    public:
+      DileptonOSSFInvariantMassWindowSelectorAlg(const std::string &name, ISvcLocator *pSvcLocator);
+      virtual StatusCode initialize() override;
+      virtual StatusCode execute() override;
+
+    private:
+      Gaudi::Property<float> m_mll_lower {this, "lowMll", 0., "MLL > LOW (in MeV)"};
+      Gaudi::Property<float> m_mll_upper {this, "highMll", 0., "MLL < High (in MeV)"};
+      /// whether to veto events instead of selecting them
+      Gaudi::Property<bool> m_veto {this, "vetoMode", false, "switch to veto-mode"};
+
+      CP::SysListHandle m_systematicsList {this};
+      CP::SysReadHandle<xAOD::ElectronContainer> m_electronsHandle {
+        this, "electrons", "", "the electron container to use"
+      };
+      CP::SysReadSelectionHandle m_electronSelection {
+        this, "electronSelection", "", "the selection on the input electrons" 
+      };
+      CP::SysReadHandle<xAOD::MuonContainer> m_muonsHandle {
+        this, "muons", "", "the muon container to use"
+      };
+      CP::SysReadSelectionHandle m_muonSelection {
+        this, "muonSelection", "", "the selection on the input muons"
+      };
+      CP::SysReadHandle<xAOD::EventInfo> m_eventInfoHandle {
+        this, "eventInfo", "EventInfo", "the EventInfo container to read selection decisions from"
+      };
+      CP::SysReadSelectionHandle m_preselection {
+        this, "eventPreselection", "SetMe", "name of the preselection to check before applying this one"
+      };
+      CP::SysWriteSelectionHandle m_decoration {
+        this, "decorationName", "SetMe", "decoration name for the MLL selector"
+      };
+
+    }; // class
+} // namespace CP
+
+#endif // EVENT_SELECTOR_DILEPTONOSSFINVARIANTMASSWINDOWSELECTORALG_H