* FDM: Improve "pitch due to flaps" according to [1], figure 62.
* FDM: Remove "delta pitch due to weight difference" and related calculations
because it was duplicating the effect.
* Calculate the current center of gravity in meters and % MAC (/fdm/jsbsim/load).
* Finetune the positions of tanks and ammunition to match [1], table 23.
* Turn tank 0 into the negative-g compartment inside tank 1. Capacity 26l.
* Rename tanks.
* Add warning about minimum maneuvering speed to the help text.
* Bugfix: Make drop tank settings persistent between sessions again.
