[boost] [PropertyMap] proposing compose_property_map

Hi. Using boost graph, and particulary reverse_graph, I needed a compose_property_map adaptor. As it may be interesting to everyone, I propose my code. http://texitoi.eu/~texitoi/compose_property_map/ I had problems to find a good return type for the get() free function. The solution proposed here has the drawback of needing a copy constructible type for the value_type of the FPMap in the concept checks. I didn't find any other type for the get() function that pass the concept checks and that do not return stack variable, and avoid useless copy. The patch may not work completely as I am discovering the boost build system. I am open to any comment. Thanks.