Add features for thresholding an image.

	* mln/fun/v2v/threshold.hh: New.
	* mln/level/threshold.hh: New.

	* mln/trait/concrete.hh: Add mln_concrete_ch_value.
	* mln/accu/tuple.hh: New.
	* tests/accu/Makefile.am: Add the following test.
	* tests/accu/tuple.cc: New.

	* mln/accu/median.hh: Add FIXME.
	* sandbox/nivault/extract_score.cc: Begin stats.

git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@1692 4aad255d-cdde-0310-9447-f3009e2ae8c0