Test how to define derived associated types in bbox hierarchy.

	* oln/core/abstract/bbox.hh (ext_vtype_): New definitions of
	fwd_piter_type and bkd_piter_type.
	(~bbox): Comment possible recursive code.
	* oln/core/gen/bbox.hh (vtypes_): Comment former definitions of
	fwd_piter_type and bkd_piter_type.
	* oln/core/abstract/pset.hh (ext_vtype_): New definition for grid.
	(fwd_piter_type, bkd_piter_type): Comment because they are now
	externally defined.



git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@519 4aad255d-cdde-0310-9447-f3009e2ae8c0