Take the max of channel A or B waveform length. Closes #11.

--CAR and BRJ
Signed integer data is asymmetric: can go to -8192 but only up to 8191

--CAR and BRJ
Lots of improvements to DAC BIST

* return all BIST values in allocated data
* test each bit line in Julia
* move DAC register addresses into constants
* set the controller clock divider
* rewrite the DAC registers for SD and clock divider after resetting in BIST