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.
Please pack all the parameters in the comments.  At this point they separate the useful comments from the code by about 10 lines and this severely degrades readability.  Same comment for every function.