Refactored parts needed for Blackoil in autodiff to get rid of UG dependency.

This patch refactors (hopefully) all parts of opm-core that are needed
by the fully implicite black oil solver in opm-autodiff and that inherently
relied on UnstructuredGrid.

We added a new simple grid interface consisting out of free functions
that will allow us to use CpGrid without copying it to an UnstructuredGrid
by the means of the GridAdapter. Using this interface we have add methods that
allow specifying the grid information (global_cell, cartdims, etc.) wherever
possible to prevent introducing grid parameters for the type of the grid.
Unfortunately this was not possible everywhere.

opm/core/grid/GridHelpers.cpp

@@ -0,0 +1,66 @@
+#include "config.h"
+#include <opm/core/grid/GridHelpers.hpp>
+namespace Opm
+{
+namespace UgGridHelpers
+{
+int numCells(const UnstructuredGrid& grid)
+{
+    return grid.number_of_cells;
+}
+
+int numFaces(const UnstructuredGrid& grid)
+{
+    return grid.number_of_faces;
+}
+int dimensions(const UnstructuredGrid& grid)
+{
+    return grid.dimensions;
+}
+const int* globalCell(const UnstructuredGrid& grid)
+{
+    return grid.global_cell;
+}
+
+const int* cartDims(const UnstructuredGrid& grid)
+{
+    return grid.cartdims;
+}
+                    
+const double* beginCellCentroids(const UnstructuredGrid& grid)
+{
+    return grid.cell_centroids;
+}
+
+double cellCentroidCoordinate(const UnstructuredGrid& grid, int cell_index,
+                                 int coordinate)
+{
+    return grid.cell_centroids[grid.dimensions*cell_index+coordinate];
+}
+
+const double* beginFaceCentroids(const UnstructuredGrid& grid)
+{
+    return grid.face_centroids;
+}
+
+const double* faceCentroid(const UnstructuredGrid& grid, int face_index)
+{
+    return grid.face_centroids+face_index*grid.dimensions;
+}
+
+SparseTableView cell2Faces(const UnstructuredGrid& grid)
+{
+    return SparseTableView(grid.cell_faces, grid.cell_facepos, numCells(grid));
+}
+
+double cellVolume(const UnstructuredGrid& grid, int cell_index)
+{
+    return grid.cell_volumes[cell_index];
+}
+
+FaceCellTraits<UnstructuredGrid>::Type faceCells(const UnstructuredGrid& grid)
+{
+    return FaceCellsProxy(grid);
+}
+}
+}