[evlatests] Sampler histograms

[Barry Clark]

> I have modified one of Mike Revnell's test programs to request a series of 
> data chunks from the evla deformatters (D351s).  From these data I've 
> created histograms showing the frequency of occurance of the various 
> sampler levels.  I took no real care to make sure attenuator levels were 
> set to be sane, but in all cases I got reasonable looking RMS values.

This doesn't matter much because it is much below the RMS level we want to
work at.  But we must remember to check at an early date the extent the 
equivalent phenomenon is showing up in the 3 bit samplers, where it can be
a killer.