Cw mode fix

[wutje]

This PR fixes CW mode by making sure the RF output mux is not switched during measurements. This is done by adding a "measurement mode":

• REFL and THRU only, this does not switch the RF output mux and creates a true CW mode
• REFL, THRU. REFERENCE, this switches RF output but less frequently, useful for doing quick measurements
• Full, this is the old behaviour measuring ECAL as well.

The signal generators are no longer updated if the frequency is equal to the last supplied, this to prevent short drops during the PLL update.

The "CW mode" is selected automatically when selecting CW Freq or a zero span.

The 3 modes are available as manual options as well. The selected mode is saved with the calibration. Note that no matter what option you select it will only take effect AFTER the initial two ECAL runs are done, this to ensure ECAL is available if later on it is selected.

[wutje]

Forgot to mention that this fixes #3