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 tried, `:%s/\" << '\\n'/\\n\"/`, but it looks a little odd when the next line is
```cpp
<< "from : " << trajectories.front()->last_time() << '\n'
```
Leaving it like that for now.