added a signature() function to NetworkSet
added a signature() function to NetworkSet

this function provides a way to visualize the relative differences
between the Network's of a NetworkSet

hohe/scikit-rf,jhillairet/scikit-rf,temmeand/scikit-rf,willhaines/scikit-rf,willhaines/scikit-rf,aerialist/scikit-rf,scikit-rf/scikit-rf,hohe/scikit-rf,jhillairet/scikit-rf,hohe/scikit-rf,Ttl/scikit-rf,gibbons/scikit-rf,Ttl/scikit-rf,temmeand/scikit-rf,Ttl/scikit-rf,temmeand/scikit-rf,scikit-rf/scikit-rf,hohe/scikit-rf,jhillairet/scikit-rf,gibbons/scikit-rf,aerialist/scikit-rf,willhaines/scikit-rf,willhaines/scikit-rf,Ttl/scikit-rf,gibbons/scikit-rf,aerialist/scikit-rf