Do not allow weighted computations in minimum_st_edge_cut.

All connectivity and cut algorithms are supposed to work on unit
capacity networks. minimum_st_edge_cut was the only exception, because
it uses the new interface to flow algorithms, is potentially able to compute
weighted cutsets. However, this complicated the implementation and could
result in a NetworkXUnbounded exception if an user used as argument a
graph with some but not all edges with an attribute called capacity. Since
minimum_cut computes weighted cuts, there is no need to duplicate
functionality here.
Improve generation of auxiliary digraph for edge connectivity.

Always build the auxiliary network and simplify the process of
building of the auxiliary digraph.