Add boolean flag to determine whether crossflow is allowed in a well

This PR adds allow_cf to the wells structure that determine whether
crossflow is allowed or not. An extra argument is added to addWell(..)
to specify the allow_cf flag.

opm/core/wells/WellsManager.hpp

@@ -38,6 +38,7 @@ namespace Opm
     struct WellData
     {
         WellType type;
+        bool allowCrossFlow;
         // WellControlType control;
         // double target;
         double reference_bhp_depth;