The function directed() no longer exists, is_directed() should be used
instead.

This includes the function motif_significance() into the clustering
module.

This implements an efficient motif counting algorithm.

Reduce duplication of code for loading C++ modules.

Also, get rid of g.underlying_graph(), and rely on "private" variable
g._Graph__graph instead.

A new 'clustering' sub-module was created.

The whole histogram code has been redone, and the code has been
simplified. The three-point vertex-edge-vertex correlation has been
scrapped, since it's not frequently used, and would make compilation
even more expensive.

This also adds some missing files to the generation routine.