* mln/topo/is_not_end_point.hh: s/nbh/nbh_fg/.
	(is_not_end_point<I, N>::is_not_end_point(const Neighborhood<N>&)):
	New ctor.
	(is_not_end_point<I, N>::set_image): New method.
	(is_not_end_point<I, N>::ima_): Turn this reference into a pointer.
	Adjust.
	* mln/topo/is_simple_point2d.hh
	(mln::topo::is_simple_point2d<I, N>::operator())
	* mln/topo/detach_point.hh
	(mln::topo::detach_point<I>::operator()):
	Add preconditions.
	* apps/generic-skel/image2d-skel-with-end-points.cc:
	Use `mln::topo::is_simple_point2d' and `mln:: topo::detach_point'
	from mln/topo/, instead of `is_simple_2d' and `detach' from
	image2d-skel.hh
	* apps/generic-skel/image2d-skel-unconstrained.cc: Likewise.
	Remove the (lack of) constraint, as
	mln::topo::skeleton::breadth_first_thinning already takes care of
	this.
	* apps/generic-skel/image2d-skel.hh: Remove.
	* apps/generic-skel/Makefile.am
	(image2d_skel_unconstrained_SOURCES)
	(image2d_skel_with_end_points_SOURCES):
	Remove image2d-skel.hh.