Iterator factorization, ndimage iterator uses nested_loop_iterator.
Add some specific functions to image2d.

	* mln/core/ndimage.hpp,
	* mln/core/ndimage.i,
	* mln/core/ndimage_iter.hpp,
	* mln/core/ndimage_pixel.hpp,
	* mln/core/ndimage_pixel_iterator.hpp: Move to...
	* mln/core/image/ndimage.hpp,
	* mln/core/image/ndimage_iter.hpp,
	* mln/core/image/ndimage_pixel.hpp,
	* mln/core/image/ndimage_pixel_iterator.hpp.
	* mln/core/image/image2d.hpp : New
	* mln/core/image/internal/nested_loop_iterator.hpp:  New