Start running a DAC waveform playback schedule. The actual hardware schedule
will be initiated by the next RegWr* command, and then the first waveform sample
will hit the DACs after the scheduleOnset specified in SetDacSchedule.
Note that every call to StartDacSchedule must be preceeded by a call to
SetDacSchedule (ie: multiple calls to StartDacSchedule each require their own
call to SetDacSchedule)).
See DatapixxDac*Demo files for examples.