ENH: A function to plot the voltage profile of a network

[alihamdan]

The idea is to have a graphic representation of the voltage levels in the whole network. We start from an initial node and walk the graph of the network from this node. The user will be able to choose the initial node, by default we choose a bus having a voltage source. The resulting graphic would show the voltage level on the y-axis in p.u. (or in percent) and the distance on the x-axis (in km).