Add direct signal-generator waveform I2C control...

Add direct control of the signal-generator waveform voltage and
frequency using remote I2C commands.

Known bugs
==========

 - Certain floating-point values result in an effective value of zero
   being set.  Adding a small value _(e.g., 0.001)_ can work around this
   issue.
 - Currently, the remote accessor methods `signal_waveform_voltage` and
   `signal_waveform_frequency` do not work.  They always return a value
   of zero.