added associated test to transmission line function to increase coverage.
added associated test to transmission line function to increase coverage.

jhillairet/scikit-rf,temmeand/scikit-rf,scikit-rf/scikit-rf,temmeand/scikit-rf,temmeand/scikit-rf,scikit-rf/scikit-rf,jhillairet/scikit-rf,jhillairet/scikit-rf