Call levelGridView instead of the deprecated levelView method.
(cherry picked from commit b7ee2548b20ee32a0e843c7d9e87078837e4a8c6)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>

Conflicts:
	dune/grid/io/file/test/conformvolumevtktest.cc
	dune/grid/io/file/test/gnuplottest.cc
	dune/grid/io/file/test/nonconformboundaryvtktest.cc
	dune/grid/io/file/test/vtksequencetest.cc
	dune/grid/io/file/test/vtktest.cc