Merge pull request #13 from pmar/insar_issues

InSAR update for 'insar stack overview' and 'external dem' issues
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.