Bugfix: Frequencies always scaled by 1e9

Fixes a frequency scaling bug when a network is created using property setters. Network(f=[1,2,3],s=[1,2,3],z0=50,f_unit='hz') results in a Network object with a Frequency property spanning 1..3 GHz because the unit was not read in during construction until after the frequency array was already scaled by the default 'ghz' unit scaling.