This fixes issue #197

This fixes a bug where equal edge descriptors would be considered
unequal. Additionally, now Vertex and Edges can be ordered (via >, <,
>=, <=).

This fixes issue #190

The new format fully supports all property map types present in
graph-tool and should be much faster and produce smaller files than the
other text-based formats.

This fixes #177.

This fixes #170

This also clarifies the relevant documentation.

This also implements direct manipulation of OpenMP settings (schedule
and number of threads) via python.

Now, if either vertex or edge filtering is enabled, simultaneous
filtering is activated. By not considering vertex or edge filtering in
completely independent fashion, the compile time memory requirements
reduced to about 50%.