Fix for getGeoPos() of AsterElevationModel

AsterElevationModel.getGeoPos() now correctly computes Geographic
coordinates as a function of pixel index for ASTER DEM. This bug was
effecting correct use of ASTER DEM in InSAR modules, and modules
using Delaunay interpolation.

All InSAR tests passing.
Merge pull request #14 from pmar/insar_issues

Fix for correct handling of ASTER in InSAR modules and for Delaunay interpolation