use zlib instead of npz for better web compatibility
Jonas requested zlib instead of npz format.
It should be OK since it is a builtin python package. A quick test indicates that PV windows binaries have not problem using it. Also easy to compress/decompress since we always know the type of each raster.
Try and swap out npz and see what effect on perf/space. All code changes should be localized to raster_wrangler.