Make BlackoilWellModel::hasTHPConstraints() work correctly in parallel.

Previously, this method did not take parallelism into account but just
checked the local wells for the constraints. Depending on the load balancing
of the wells this sometimes led to different return values of the function on different
processors. As the output is used to limit the time step size, different processors
were sometimes using different time steps in their local computations. This screwed
up convergence checks int the nonlinear operator such that only some processor thought
convergence was already achieved while others wanted to do more iterations.

With this commit the method now returns whether there is any well on any processor
with the constraint being true.

opm/autodiff/BlackoilWellModel_impl.hpp

@@ -122,12 +122,13 @@ namespace Opm {
     BlackoilWellModel<TypeTag>::
     hasTHPConstraints() const
     {
+        int local_result = false;
         for (const auto& well : well_container_) {
             if (well->wellHasTHPConstraints()) {
-                return true;
+                local_result=true;
             }
         }
-        return false;
+        return grid().comm().max(local_result);
     }