[boost] [property-map] Generalizing identity_property_map for arbitrary types

Hi, I've found the property map library to be very useful and would like to propose a minor enhancement to permit the identity_property_map to work for arbitrary types. The reason for this is that some computationally intensive algorithms suffer noticeable performance degradation when I convert them to generic algorithms using property maps. Having a typed identity property map that returns, for example, the double passed into it significantly speeds up our algorithms, at least for those fields that are not mutable. I've attached a patch, and could supply a motivating example, if anyone is interested. -- Noel