Fix for issue 1394 (YASim's geometry for non-cylindrical fuselages is wrong)

Fix for issue 1394 (YASim's geometry for non-cylindrical fuselages is wrong)

Correct handling of fuselage's "midpoint" XML attribute so that the midpoint's
location matches the point assumed by all aircraft developers and YASim docs.
This means the fuselage won't be generated "back-to-front".
Also correct the variation of diameter for tapered fuselages so that both ends
of the fuselage narrow from the midpoint to the endpoints.
Before this, one end was narrowing from the endpoint to the midpoint, the
opposite of what was expected.
These changes affect both weight distribution and the distribution of lateral
aerodynamic forces.