ENH: Adding a 2D geoview. Adding the geographic projection library libproj4
to Utilities. Updating the architecture of the geospatial views. All multi-resolution sources are now subclasses of vtkGeoSource. Each source has its own worker thread for fetching refined images or geometry. On the 3D side, vtkGeoGlobeSource is an appropriate source for vtkGeoTerrain, and vtkGeoAlignedImageSource is an appropriate source for vtkGeoAlignedImageRepresentation. On the 2D side, vtkGeoProjectionSource is an appropriate source for vtkGeoTerrain2D, and the image source is the same.