correct the nports calculation
correct the nports calculation

Correct the nports according to proposal from @cloidnerux in issue #366
temmeand/scikit-rf,temmeand/scikit-rf,scikit-rf/scikit-rf,scikit-rf/scikit-rf,jhillairet/scikit-rf,temmeand/scikit-rf,jhillairet/scikit-rf,jhillairet/scikit-rf