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 am uncomfortable with having this as a separate member, as it could easily become inconsistent with the trajectories.  I would make it a function: in opt mode it would just grab the time from the first trajectory (that of the sun?) and in dbg mode it could check for consistency.