Added graph property support to read_graphml

Graph properties from graphml files were never read. After this fix, the graphml reader uses the graph properties of the first top-level graph. The graph properties of the other top-level and all nested graphs are ignored.
I also added testing of reading and writing of graph properties to graphml_test.cpp (and graphml_test.xml). The other changes in graphml_test.cpp are: changing assert to BOOST_CHECK and tolerance to small rounding errors while comparing properties of type double.