This improves per formance, since the conversion is not performed at
every drawing operation.

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.