Move GridManager to grid subdir.

Also remove GridAdapter (moved to dune-cornerpoint), and
moved grid.c implementation file to grid subdir.

tutorials/tutorial3.cpp

@@ -28,7 +28,7 @@
 #include <vector>
 #include <cassert>
 #include <opm/core/grid.h>
-#include <opm/core/GridManager.hpp>
+#include <opm/core/grid/GridManager.hpp>
 #include <opm/core/io/vtk/writeVtkData.hpp>
 #include <opm/core/linalg/LinearSolverUmfpack.hpp>
 #include <opm/core/pressure/IncompTpfa.hpp>