added checks/tests to network
added checks/tests to network

added some functions for common tests and checks. the tests
return boolean values, while the checks usually raise exceptions.

i also noticed what apears to be a major bug in the connect()
function for non-matching port impedances. i will have to
create tests for this using some kind `truth` such as ads or
similar.

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