Take advantage of BOOST_CONCEPT_USAGE for defining usage requirements:

Using this macro is better than simply defining a destructor because
usage patterns will be tested whenever associated types are accessed.


[SVN r35199]

libs/graph

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-Subproject commit 04fea004d91d20faa276877663ee316d208d5b7c
+Subproject commit 9233499dbc46cba58655f73ea8e83f06b10cd393

libs/iterator

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-Subproject commit 7243852b0a6e96371124bc35ae65a8d48ee238bc
+Subproject commit 71abf15b6001eab1e8a4243d996f01f07e5d01f4