fixed bug in gen_three_port
fixed bug in gen_three_port

s33 was assigned incorrectly
now handles both "12" and "21" names properly

also added functionality for 1,2 or 3 inputs

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