Pulse with jitter

Level-2 m-code S-Function that acts as a pulse generator with jitter around a nominal period.

The user specifies
1) a nominal period for the pulse (i.e. the nominal time between rising pulses)
2) the time that the pulse may be (randomly) delayed or advanced at each time step. The block assumes a nominal 50% duty cycle, hence this must be less than 1/4 of the nominal period.
3) whether the pulse is initially high or low.