[igraph] Rewire in general

[Katharina Zweig]

Hi Tamás,

thanks for the work all of you put into igraph. 

I was just reading some requests to change the rewire-function: I am also in
trouble because I need a fast rewiring method that maintains the (directed)
degree distribution and AVOIDS self-loops. I noticed that in November 2012
somebody actually wanted the rewiring to include self-loops, and you fixed that
'bug', but actually I do not think it is a bug if self-loops cannot be created.
In my opinion, it would be very nice to have a Boolean that allows switching the
creation of self-loops on or off. (In case self-loops are wanted, the fasted way
would be to use r2dtables, as they easily create matrices with fixed marginals
and self-loops).  In any case it would be helpful if the way you handle
self-loops would become more clear. 

Another thing I noticed: The verbal description of the rewire-function does not
make clear whether the permutation of undirected graphs is correct, i.e.,
whether edges can be "picked" in both directions. A small test on the rewire
function quickly revealed that this is actually the case, but maybe the
documentation could be a bit more verbose there. 

Thanks again,
Nina