Refine MinTargetDistance, and the ClosestApproach computation

For vessel encounters.  Updates to CelestialBody encounters is still to-do.
Eliminate OrbitExtensions.SwapYZ and MathExtensions.Reorder

The latter is only used by the former, and the former can be accomplished by the .xzy operator that is part of Vector3d.  No sense making a couple of function calls and a switch statement when we can just get it for "free".
Merge pull request #82 from MOARdV/master

Improve min distance computation