Avoid subprocess.PIPE in long-running processes

* It gets saturated and locks the process if the subprocess has too much
  output.
Change sample batching sizes

* It is now optimizes for fewer, slower samples.  Running fast models
  with these settings will be frustratingly slow.
Avoid automatic selection of repo/rev/model

* Because sampling is very long-running now, we don't want to
  auto-trigger on load
* Adding "none" options means the "select" handler works correctly
Calculate the number of displayed samples

* For long-running sampling processes, the number of samples displayed
  is too large, so only use as many as pixels will display.
Add Processing.js display of bezier samples

* Split UI area in half
* Copy Processing sequence from bezier.ScribbleDraw
* Trigger Processing rendering when receiving new sample points and when
  selecting points on the streamgraph