added caled_ntwks property, and modified apply_cal()
added caled_ntwks property, and modified apply_cal()

caled_ntwks returns the calibrated calibration standards

i modified the apply_cal() function to take lists of networks
as well.

Ttl/scikit-rf,aerialist/scikit-rf,jhillairet/scikit-rf,hohe/scikit-rf,temmeand/scikit-rf,Ttl/scikit-rf,scikit-rf/scikit-rf,Ttl/scikit-rf,hohe/scikit-rf,willhaines/scikit-rf,gibbons/scikit-rf,aerialist/scikit-rf,gibbons/scikit-rf,temmeand/scikit-rf,jhillairet/scikit-rf,willhaines/scikit-rf,aerialist/scikit-rf,gibbons/scikit-rf,hohe/scikit-rf,temmeand/scikit-rf,hohe/scikit-rf,willhaines/scikit-rf,Ttl/scikit-rf,jhillairet/scikit-rf,scikit-rf/scikit-rf,willhaines/scikit-rf