* headers.mk: add new header to distribution.

	* mln/accu/all.hh: include new header.

	* mln/accu/rms.hh: Compute the root mean square.

	* tests/unit_test/Makefile.am,
	* tests/unit_test/mln_accu_rms.cc: add unit test.

git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@3239 4aad255d-cdde-0310-9447-f3009e2ae8c0