Add operator- to n-dimensional dpoint types.
	
	* oln/core/traits_id.hh (op_uminus_dpointnd): New.
	* oln/core/abstract/dpoint_nd.hh (operator-): New.
	(impl_less): New.
	(case_): New case for operator-.
	* oln/core/2d/dpoint2d.hh (dpoint2d_): New ctor.



git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@544 4aad255d-cdde-0310-9447-f3009e2ae8c0