Switched back the behavior of looking for fewer links than are available
to simply use all that are present. This means that the P option will
not throw a nan for sparse networks anymore.
Bug fix of previous link counting change.
Added option to in directed_spr to average together the weights of a
node's links.
Improved unit testing of average_weight_by_node
Yet better unit testing of average_weight_by_node, and bug fix of the
implementation in richclub.py