Improve NISTMultilineTRL accuracy
Improve NISTMultilineTRL accuracy

Fixes incorrect equation that gave inaccurate values when measurements were
noisy.

Adds a method for determining R1R2 coefficients given in the
"Multiline TRL revealed" paper. Method to be used can now be chosen with
'k_method' argument.

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