Add array2d and tracked_ptr.
	
	* oln/core/2d/array2d.hh: New.
	This file also provides the tracked_ptr class which should be
	moved later in a more appropriate file.
	* oln/core/2d/image2d.hh (lvalue_type, is_mutable_type): Add.
	(data_): New attribute.
	(impl_op_read): Update; code is no more fake.
	(impl_op_readwrite): New.
	(image2d): Update.
	* oln/Makefile.am (nobase_oln_HEADERS): Update; remove
	the core/2d/grid2d.hh duplicate.



git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@578 4aad255d-cdde-0310-9447-f3009e2ae8c0