Compute well potentials

The well potentials are caculated based on the well rates and pressure
drawdown at every time step. They are used to calculate default guide
rates used in group controlled wells.

well_perforation_pressure_diffs is stored in
WellStateFullyImplicitBlackoil as it is needed in the well potential
calculations.

opm/autodiff/SimulatorBase.hpp

@@ -157,6 +157,13 @@ namespace Opm
                     const BlackoilState&            x,
                     WellState& xw);
 
+        void computeWellPotentials(const std::size_t               step,
+                                   const Wells*                    wells,
+                                   const BlackoilState&            x,
+                                   const WellState& xw,
+                                   std::vector<double>& well_potentials);
+
+
         // Data.
         typedef RateConverter::
         SurfaceToReservoirVoidage< BlackoilPropsAdInterface,