ADD: added error_ntwk support for two-port 7-term error model.
ADD: added error_ntwk support for two-port 7-term error model.

note that these two networks represent the hardware involved, but
cannot by themselves be used for de-embedding

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