Use pore volume weighted averaged hydrocarbon state in rateConverted.

- pressure, rs and rv is averaged using hydrocarbon pore volume weights.
- pvtRegions is used as input in the conversion factor calculations.
- the pvt cell of the first well cell is used as the pvt index.
(Completing a well in two different PVT regions sounds like a very bad
idea anyway)
- FIP region support is added to the rate converter also for the ebos
interface.

opm/autodiff/WellInterface.hpp

@@ -94,6 +94,9 @@ namespace Opm
         /// Well name.
         const std::string& name() const;
 
+        /// Well cells.
+        const std::vector<int>& cells() {return well_cells_; }
+
         /// Well type, INJECTOR or PRODUCER.
         WellType wellType() const;