* dhc2-base.xml (payload): new; expose the various weights and connect them to the FDM.
(limits): new; document the max takeoff weight.
* dhc2F.xml: add pointmasses for the floats. Remove the default fuel
load of the wingtip tanks, which are rarely used IRL.
(emptywt): correct.
* dhc2W.xml: ditto. Also, reduce the differences between the F and W
FDMs to the bare minimum.
