fixed error when calibration NIST_TRL with no switch terms
fixed error when calibration NIST_TRL with no switch terms

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