[NB] Add first draft of feedback measurement...

Add first draft of feedback measurement.

Simplify and re-factor the feedback measurement to share code between
measuring high-voltage and feed-back voltage, where appropriate.  This
includes adding the following:

 - `PeakToPeakMeasurement` class.
 - `DMFControlBoard::measure` method.

See source for details.

Known issues
============

 - The Python control-board API makes one call per sample when reading
   voltage samples from the control-board, since array transfers are not
   currently supported by the Protocol Buffers RPC back-end.
 - Due to [a bug in Arduino Protocol Buffers serialization][1], setting
   the waveform voltage to certain floating point values results in the
   actual value being set to near-zero.  To compensate for this issue,
   when attempting to set the waveform voltage for automatic gain
   adjustment, if the resulting voltage is incorrect, a small
   delta-voltage is added to the voltage until the desired voltage is
   reached.

TODO
====

 - Once the Protocol Buffers back-end supports array transfers, return
   many measured samples at once, rather than one sample at a time.