Fix reference calibration...

Only instantiate an `AgilentOscope` when needed.  Otherwise, the control
board can be incorrectly identified as a `VISA` device, preventing the
control board firmware from connecting properly.
Make reference setup diagram smaller
Fix impedance calibration...

- Turn on auto amplifier gain.
- Set waveform voltage to 50V to auto-adjust gain.
- Add debug output print statements to display
the settings for each measurement step.