make IHiveWhiteBoard::getNumberOfStores() const

See merge request !229

GaudiHive/src/HiveWhiteBoard.cpp

@@ -464,7 +464,7 @@ public:
   }
 
   /// Get the number of event slots (copies of DataSvc objects).
-  size_t getNumberOfStores() override { return m_slots; }
+  size_t getNumberOfStores() const override { return m_slots; }
 
   /// Get the list of new DataObjects in the current store.
   StatusCode getNewDataObjects( DataObjIDColl& products ) override

GaudiKernel/GaudiKernel/IHiveWhiteBoard.h

@@ -45,7 +45,7 @@ public:
    * 
    * @return Number of event stores allocated in the whiteboard
    */
-  virtual size_t getNumberOfStores() = 0;
+  virtual size_t getNumberOfStores() const = 0;
   
   /** Get the latest new data objects registred in store.
    *