no conversion from type to const_type

[ksvanhorn]

Apparently there is no conversion from
boost::property_map<Graph, Tag>::type to
boost::property_map<Graph, Tag>::const_type, contrary
to what anyone experienced with conversions of
T& to T const &, T * to T const *, and mutable to
immutable iterators would expect.

[Douglas Gregor]

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This actually turns out to be a bit hard, if only because we didn't do it in 
the first place. I'm postponing this and adding it to the challenge list.

[Markus Schöpflin]

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Closing the bug report as the request has been added to the
challenges list of the BGL.