Compute phase pressures in subset of cells

This commit adds support for assigning the initial phase pressure
distribution to a subset of the total grid cells.  This is needed in
order to fully support equilibration regions.  The existing region
support (template parameter 'Region' in function 'phasePressures()')
was only used/needed to define PVT property (specifically, the fluid
phase density) calculator pertaining to a particular equilibration
region.

opm/core/simulator/initStateEquil.hpp

@@ -160,10 +160,11 @@ namespace Opm
             PhaseUsage  pu_;
         };
 
-        template <class Region>
+        template <class Region, class CellRange>
         std::vector< std::vector<double> >
         phasePressures(const UnstructuredGrid& G,
                        const Region&           reg,
+                       const CellRange&        cells,
                        const double            grav = unit::gravity);
     } // namespace equil
 } // namespace Opm