Add live_view module to qutil.plotting.

The live_view module provides a matplotlib interface for concurrent data viewing using animations and blitting. For line plots without axis rescaling, frame rates of ~100 fps can be reached. A 1080x1920 pixel image can be displayed at ~20 fps. Data sources are pushed to a background thread from where they can fill a queue.

See mjolnir commits a771a5cc..205318e6 for earlier history.

qutil/plotting/live_view.py

@@ -953,7 +953,7 @@ class LiveViewBase:
             # Thread not started
             pass
 
-        if mp.current_process() is not mp.main_process:
+        if mp.parent_process() is not None:
             # Running in a separate process. Since we cannot restart the view
             # from there, we only stop data production so that LiveViewProxy.attach()
             # can be used to plot new data