Added Sector. Minor changes to Location.

git-svn-id: http://worldwind31.arc.nasa.gov/svn/trunk@2460 822214bb-dcf7-4c8d-a767-17ae0fb20d88
Implemented ZeroElevationModel.

git-svn-id: http://worldwind31.arc.nasa.gov/svn/trunk@2461 822214bb-dcf7-4c8d-a767-17ae0fb20d88