fix NetworkSet.from_zip
fix NetworkSet.from_zip

- convert ZipExtFile to StringIO to be read by the touchstone parser
- discovered that NetworkSet doesn't have dedicated tests... -> next step
scikit-rf/scikit-rf,temmeand/scikit-rf,jhillairet/scikit-rf,jhillairet/scikit-rf,temmeand/scikit-rf,scikit-rf/scikit-rf,temmeand/scikit-rf,jhillairet/scikit-rf