adding leastStrictBhpFromBhpLimits()

to pick the least strict bhp limits in the well controls. It is used to
calculate the well potential when there is no thp limits there.

If there is thp limits there, it will be used for further chosing of the
bhp value for well potential calculation.

opm/autodiff/StandardWellsDense.hpp

@@ -374,6 +374,7 @@ enum WellVariablePositions {
                                     const int well_index,
                                     std::vector<double>& well_flux) const;
 
+            double leastStrictBhpFromBhpLimits(const int well_index) const;
         };