GeoRaster.getWindow lazy

[guydou]

I think GeoRaster.getWindow should not materialize the data of the raster in case the raster is off memory.

I think using VRT we can serialize the raster to get a specific window.

It is important to do that, so we could create GeoRasters that points to windows, these raster will behave the same as they behave today,
but we could also create list of windows and distribute it later to run apply_transform on a cluster of resources, using today code that will casue actually reading the data