modified the "graphing logic" of the class. now each topology has to
overload de graph function. tree-like topos are drawn as trees and
random-like topos are drawn with a circular layout.
now the graphs are drawn using networkx's representation/view of the
topology as oposed to mininet's view. This means that the hosts are
not drawn (but you can be assured that there is a host for each switch)
and all switches are now red.