adding a non-const wellCollection() in WellsManager.

For the WellModel from the simulator to use. Not decided totally,
    well_collection might need to be updated during the simualtion due
    to the update the target of wells.

opm/core/wells/WellsManager.cpp

@@ -373,6 +373,12 @@ namespace Opm
         return well_collection_;
     }
 
+    WellCollection& WellsManager::wellCollection() {
+        return well_collection_;
+
+    }
+
+
     bool WellsManager::conditionsMet(const std::vector<double>& well_bhp,
                                      const std::vector<double>& well_reservoirrates_phase,
                                      const std::vector<double>& well_surfacerates_phase)

opm/core/wells/WellsManager.hpp

@@ -114,6 +114,7 @@ namespace Opm
 
         /// Access the well group hierarchy.
         const WellCollection& wellCollection() const;
+        WellCollection& wellCollection();
 
         /// Checks if each condition is met, applies well controls where needed
         /// (that is, it either changes the active control of violating wells, or shuts