ADD: working PNA and writing doc-strings.
DOC: vi.vna.PNA docs
CHG: mathfunctions.magntiude to dB, removed isneginf switch, need to
	test this better
ADD: skrf icon
TODO: need to setup tests for NetworkSet.
BUG: NetworkSet pickling doesnt work for dynamically generated
	methods. but Network.plot* does. need to fix this
ADD: network: new function to combine 4 oneports into a two port
ADD: network: function to operate on each parameters per frequency slice
CHG: vi working on PNA vi