Fixed Flight Computer for vessels without Reaction Wheels.

This fix consists basically only of a gimbal vectoring calculation update. The old code seemed to be from an earlier version of KSP.
The bug manifested itself in a craft that would not cahnge orientation whatsoever when the Flight computer was enabled. It only started to jerk around and kept facing the orientation it was facing in. This was independent of the mode of the Flight Computer. The bug can be easily reproduced by adding the following .cfg file and using the Flight Computer on the Stayputnik probe core:

    @PART[probeCoreSphere]
    {
      !MODULE[ModuleReactionWheel]
      {}
    }

I encountered this bug while playing with the RealismOverhaul mod, which removes from the reaction wheels from essentially all early tech probes.

I also fixed a divide by zero bug that crashed KSP, when using the above setup.

No mod other than RT2 and the custom .cfg file mentioned above are required to reproduce the bug.
Agreed that all the functions could do with proper specs on what they're for. Unless @Cilph or @erendrake say otherwise, though, I'm pretty confident in defining `Inverse()` as what the original code said: the transformation (x, y, z) -> (1/x, 1/y, 1/z) of a three-element vector.

The reason I don't like returning the input is that `Inverse()` gets called in more than one place. If you're simply inverting the available torque around each axis, then your solution should be adequate (the flight computer will simply misjudge the magnitude of any slew commands). For things like the moment of inertia, things are messier. Same goes for unforeseen additions of `Inverse()` calls in the future.

I'll look into exactly where it gets called and see if there's a solution that makes us all happy, then edit this post accordingly.