Let WellModel decide whether to apply well contributions.

It queries the Well whether the jacobian also contains well contributions.
If not then it applies them in the operator in addition. Thus the
well knows whether that is needed or not.

opm/autodiff/WellInterface_impl.hpp

@@ -844,14 +844,6 @@ namespace Opm
         return 1.0;
     }
 
-    template<typename TypeTag>
-    void
-    WellInterface<TypeTag>::addWellContributions(Mat& mat) const
-    {
-        OPM_THROW(NotImplemented, "This well class does not support adding well contributions"
-                  << "to the matrix");     
-    }
-