Add ADC16#zdok_rev

Returns the ZDOK pinout revision for which the ADC16 design was built.

ZDOK pinout revision 1 requires the use of an extra ribbon cable to
program the ADCs.  ZDOK pinout revision 2 programs the ADCs through the
ZDOK connector.
Display ZDOK revision in adc16_status output
Make calibrate more robust if ADC1 clock unlocked

Also reject negative chip numbers.
Continue initializing if ADC1 clock is unlocked
Only calibrate chips A-D if ADC1 clock is unlocked
Make delay tap calibration more robust

The original calibration routine kept an Array of good tap settings and
did not properly handle the case where more than one good range of tap
settings is found.  In that case, it would "give up" and leave the delay
tap set at 31.

The new calibration routine keeps an Array of Ranges of good tap
settings.  Then is finds the longest Range and sets the delay tap to the
midpoint of that range.  This is much more robust.
Tweak adc16_plot_chans histo plot

Limit max_idx and add X padding to plot
Add mean and stddev methods to ADC16Test::Histo

Both class and instance methods are added.
Bump version to 0.3.0