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.
Lines 268-272 use the word "history" with a meaning different from those of the identifiers, which is confusing.  They also use the word "synchronized", which doesn't show up in the identifiers but probably should.  This is further indication that the identifiers need revising.