Port graph I/O to new filtering engine, enable graph pickling, and fix several issues
Now graphml files properly contain all the supported value types, which are all perfectly preserved when read (floating point data is now saved in hexadecimal format). Several other improvements were made, such as the ability to read and write to python file-like objects. It is also now possible to have arbitrary python object properties, and store them persistently (which is done internally with the pickling interface). vector<bool> was totally abolished, since its implementation is quite broken. See: http://www.gotw.ca/publications/N1211.pdf and http://www.gotw.ca/publications/N1185.pdf Now a uint8_t (aka. char) is used in graph properties instead of a bool. Graph types can now be fully pickled (this may not be feasible memory-wise if the graph is too large, since the whole XML representation is dumped to a string before it is saved on disc).