ADD: added error_ntwk support for two-port 7-term error model.

note that these two networks represent the hardware involved, but
cannot by themselves be used for de-embedding
ADD: networkSet.s_dict_to_ns to support calibration coefs
ADD: added network-type representation of calibration coefficients
	to Calibration.
CHG: changed Calibration.plot_coefs_db to call a more general
	method plot_coefs, which uses the Network plotting methods
	instead of re-implementing them
ADD: working on PNA and test case