Revised g-forces

- G-forces was running/not running randomly due to clash in XEH classname with AGM_Interaction. That depended in pbo load order.
- The current G calculation was physically inacurate (and more complicated than it needed to be).
- Implemented a maximum for instantaneous accel of +- 10 G. Possible solution to #1511.

Now acceleration vector is calculated using first order accurate finite differences between new and old vel. Then it is proyected on the "vertical" axis of the vehicle (people are supposed to be aligned with that). This is a better way of doing the calculation and is simpler and more reliable.

The former version had incorrect behaviour for certain cases. E.g. if you did a fast vertical take off with a chopper it reported less than 1G, when in fact the pilot should experience more than 1G. I think this kind of things made red-outs more common than they should be.