Added logic to use North-Up views when far from the ellipsoid.
Switch to using semiMajorAxis for the North-Up decision.

Thanks to Matt Berry for the math.
semiMajorAxis can be negative for hyperbolic orbits.
Changes after review.
Merge remote-tracking branch 'origin/master' into north-up
Merge pull request #1785 from AnalyticalGraphicsInc/north-up

North-up viewing of high-orbit objects