Return intended waveform_voltage

The previous waveform_votlage() implementation returned the signal generator
voltage scaled by the amplifier gain, but this gives an incorrect value if
the amplifier gain is incorrect (e.g., if the amplifier is off).
Fix equation for x-position
Return amplifier gain and vgnd levels with measure_impedance calls
Increase default amplifier gain to 300

This makes it less likely that people will inadvertently damage their
systems.
Set default waveform frequency on startup
Merge remote-tracking branch 'github/dev' into dev

Note that this commit reverts the change in commit 4f67301 which
incorrectly modified the equation for x-position.