eflowRec: Fix memory `leak'

eflowTrackClusterLink was maintaining a pool of objects, looked up by cluster and track pointers. But these were never cleared, so these objects built up over the entire run of the reconstruction.

Changed to use a SlotSpecificObj that gets cleared after an event is finished. Also a number of other minor cleanups.

Reconstruction/eflowRec/eflowRec/eflowCellIntegrator.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 /*
@@ -125,14 +125,14 @@ public:
 
   inline void setEtaSq(double xSq) { m_etaSq = xSq; }
 
-  inline double evaluateStdExp(double rSq) { return m_norm * exp(-rSq * m_oneOverTwoSigmaSq); }
-  inline double evaluateLookupExp(double rSq) { return m_lookupExp->evaluate(rSq * m_oneOverTwoSigmaSq)*m_norm; }
+  inline double evaluateStdExp(double rSq) const { return m_norm * exp(-rSq * m_oneOverTwoSigmaSq); }
+  inline double evaluateLookupExp(double rSq) const { return m_lookupExp->evaluate(rSq * m_oneOverTwoSigmaSq)*m_norm; }
 
   /** The evaluate method for the integration. The implementation depends on the template parameter */
   inline double evaluate(double y);
 
 private:
-  eflowLookupExp* m_lookupExp;
+  const eflowLookupExp* m_lookupExp;
   double m_oneOverTwoSigmaSq;
   double m_norm;
   double m_etaSq;