This also changes the semantics of random_rewire() when parallel edges
/ self loops are forbidden, but the initial graph contains them. Now,
instead of throwing an exception, the rewiring is attempted, with no
new parallel edges / self-loops being created. Under some
circumstances, if the degree sequence is graphical, eventually the
graph can become simple after sufficiently many sweeps.

To specify fully random moves, one should set the parameter "c" to
"float(inf)", as explained in the docstring.

This fixes some strange bugs where deterministic position updates lead
to strange artifacts.

Xin Sun authored Dec 21, 2013 and Tiago Peixoto committed Dec 21, 2013

In case google sparsehash uses a non-standard prefix, e.g., in Homebrew.

The routine wrongly returned 1/lambda instead of lambda, as the leading
eigenvalue of the adjacency matrix.