incorperate the review comments/decisions for multi-region PVT

the largest change is that all classes below opm/core/props/pvt take
the PVT region index as an argument, the higher-level ones (i.e.,
BlackoilProps*) take cell indices.

opm/core/props/BlackoilPropertiesBasic.cpp

@@ -157,9 +157,11 @@ namespace Opm
     ///                    matrix A = RB^{-1} which relates z to u by z = Au. The matrices
     ///                    are assumed to be in Fortran order, and are typically the result
     ///                    of a call to the method matrix().
+    /// \param[in]  cells  The index of the grid cell of each data point.
     /// \param[out] rho    Array of nP density values, array must be valid before calling.
     void BlackoilPropertiesBasic::density(const int n,
                                           const double* A,
+                                          const int* /*cells*/,
                                           double* rho) const
     {
         const int np = numPhases();
@@ -177,7 +179,7 @@ namespace Opm
 
     /// Densities of stock components at surface conditions.
     /// \return Array of P density values.
-    const double* BlackoilPropertiesBasic::surfaceDensity() const
+    const double* BlackoilPropertiesBasic::surfaceDensity(int /*cellIdx*/) const
     {
         return pvt_.surfaceDensities();
     }