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.
Not really. What we could do is call CheckVesselInvariants in `HistoryTime()`, but some of these invariants are violated within `AdvanceTime` (`AdvanceTime` enforces them).

I think the checks currently in `HistoryTime` should be done by (friend) tests between calls to member functions, rather than in the middle of nowhere and `#ifdef`ed away, this class is complicated enough without containing its own tests.

Removing the redundant checks for now.