Gravitation

The plugin now does something (no plotting included).
It is *fast*.
Logging is also *fast*. Activating `VLOG(1)` and having it flush immediately does not noticeably affect performance. For the moment, setting `FLAGS_v = 1` and `FLAGS_logbuflevel = -1` (we don't have the flags library), since that makes tracking segfaults much easier. These flags should be made adjustable from the in-game GUI later on.
I'm not in love with the name ``rendering_extension``.  It's long and it doesn't carry the notion that this is a trajectory.  (I think that ``history`` is a bit confusing too, but to the lesser extent.  I guess that I would like these things to say that they are trajectories.)