Adds a generator for the chordal p-cycle graph.

Adds the chordal p-cycle graph, a mildly explicit algebraic construction
of a family of 3-regular expander graphs. Also, moves both the existing
expander graph generator function (for the Margulis-Gabber-Galil
expander) and the new chordal cycle graph function to a new module,
``networkx.generators.expanders``.