Need to call recalc() on the RigidBody during solution, or else it

Need to call recalc() on the RigidBody during solution, or else it
won't apply the right gross weight due to fuel differences.

When solving for zero force, do so in the global frame, not the
aircraft's.  In principle, this shouldn't matter (zero is zero in all
frames), but in practice this should help to avoid oscillations.
Calculating lift as force perpendicular to the ground (and not the
wing plane) is clearly the Right Thing, anyway.

Also added support for a /yasim/gross-weight-lbs property, which
should be generically useful.