- add "spectrum" message mimicing the spectrum element's message

- remove lots of stuff made obsolete by this change

highest SNR event within each buffer, but implement proper per-channel
threshold crossing detection with state preserved across buffers.

- update for change of "channel" tag to "channel-name"
- don't combine trigger streams, write each to a separate output file
  (temporarily, to work around a segfault in n-to-1)