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.
So this is a boolean that can only transition from ``false`` to ``true`` but not from ``true`` to ``false``, right?  It would be good to encapsulate this behavior in a small auxiliary class.