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.
I'm not a big fan of exceptions in general and not really in favor of using one in this case either. Given that this mod already uses exceptions sparsely, I'd rather search for another solution to this problem. Actually I'm not quite sure what exactly the "inverse" of a vector really should be, so my suggested fix could indeed be way off. I just tried to prevent the program from crashing. Maybe we should first define what this method is supposed to do, before searching/implementing for a solution.

The change I suggest only recovers as gracefully as it can from invalid input, e.g. when the craft has no means of applying torque along the roll axis (as mentioned in the submit message). Throwing an exceptions and thus most probably preventing the auto pilot from making any input, would break the auto pilot for vessels without reaction wheels, but at least it wouldn't crash the whole program...

So yeah, I don't know. I think the best solution would be to remove this method and fix the locations where it's called, since the callers know what's correct in case one of the arguments is zero. Your call.

Just wanted to push back a little and give some alternatives before before changing anything.