dead-time zero insertion for simulation

[guruofquality]

Consider a simulation style demo where a block produces paced bursts. We have no way to insert zeros in the dead-time between bursts so the receiver shows burst back to back. Even the Lora demo ended up using a significant amount of configurable padding to simulate this. A dead-time insertion block should have two ports:

• port 0 is the bursty data with end of frame label
• port 1 is the zero (or noise) stream

The block muxes between the two using the end frame label to switch to port 1, and the presence of data on port 0 to switch back.