2D features to a mixin class. This enables to provides basic N-d
features (e.g. indexing with a pair x, y) to any image inheriting from
image_base (i.e almost all images incl. morphers)


	* mln/core/domain/box.hpp,
	* mln/core/image/image2d.hpp,
	* mln/core/image/internal/nested_loop_iterator.hpp,
	* mln/core/image/ndimage.hpp,
	* mln/core/image/sub_image.spe.hpp,
	* mln/core/image_base.hpp: Fix
	* mln/core/image/internal/image_base_at_mixin.hpp: New