Fixed slewing inaccuracies in flight computer

Note: reposted with correct destination branch

This patch replaces the internals of the RemoteTech flight computer with the corresponding MechJeb code, including their PID controller and a manual calculation of the ship's rotational inertia (stock one gives back garbage). I've tested it briefly with both a light and a heavy ship, and it appears to slew to any direction with no wobbling or tumbling. This patch does **not** include the RCS fix in RemoteTechnologiesGroup/RemoteTech#97. At the moment, ships slewing via a mix of reaction wheels and RCS never quite settle down, likely because they're overcompensating for small errors.

In addition to ripping out the guts of `FlightCore`, this patch adds some vessel-state code to `FlightComputer`, and I'd like feedback from someone familiar with the RemoteTech codebase on whether that's the most appropriate place for it.

Test version is up at https://github.com/Starstrider42/RemoteTech/releases/tag/slew-fix-1 if you'd like to try it on a bigger variety of ships.

Fixes #76. Does NOT address #25, so they are in fact separate bugs.

Also, reduces but does not completely eliminate the "roundabout" slewing you get when you time warp to the other side of the planet.