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.
Don't know about the nulls (I assume you mean for `vessel.angularVelocity`?) but I agree that SwapYZ would be cleaner. Will do.