This fixes the expected values for several doctests, after
random_graph() re-factoring.

Small fixes of some examples in the docstrings.

This completes all the documentation up to this point.

This modified several docstrings and introduces a quick-start guide.

This adds vertex_average() and edge_average() to the stats module.

Reduce duplication of code for loading C++ modules.

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

This is to avoid a common error of not being able to correctly normalize
a histogram because of truncated integer division.

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.