The string representation is now compressed, to avoid strings of huge
length, which makes pickle.dump() bork.

Fix problem in search functions when 'zero' and 'infinity' values cannot be cleanly converted to property type

Removal of vertices now compiles and works properly.

This catches up to a previous modification where vertex/edge descriptors
need to receive a weakref to the graph class.

This fixes a problem with pickling python::object property maps.