* data/vaucanson.xsd: Split the `expressionAttributes' into two
	attribute groups: `atomAttributes' and `nodeAttributes'.
	Add left and right weights for nodes of an expression.
	Refine the drawing tag.
	* doc/xml/xml_proposal.tex: Update accordingly.
	* include/vaucanson/algebra/implementation/series/rat/xml_exp_visitor.hh,
	* include/vaucanson/algebra/implementation/series/rat/xml_exp_visitor.hxx:
	Add the support of left and right weights for output. Fix a bug	about
	weights.
	* include/vaucanson/algebra/implementation/series/rat/exp.hh,
	* include/vaucanson/algebra/implementation/series/rat/exp.hxx: Minor fix.
	* include/vaucanson/xml/tools.hxx: Add the support of left and right
	weights for input.
	* src/tests/xml/xml_label_node_tester.cc: Enhance the tests on weights.
	* src/tests/xml/label_node_ref.xml,
	* src/tests/xml/label_node_ref.dot: Update accordingly.