Input edge triggering

• Add a firmware edge-triggering capability.
  • Sample continuously, but do not buffer, until an edge crossing is detected.
  • Then buffer and push a sample frame.
  • In the absence of triggers for a while (~500 ms), push a sample frame anyway, unless in "single" mode.
• Make it configurable via USB.
• Add a "single capture" mode configurable via USB.
  • When this mode is requested, start capturing (if paused); push the next triggered frame; then stop capturing.

Some external ADCs also have "voltage alarm" features that could be usable in this capacity.