fix missing imports
add ability to use webservice to get tile urls, plus fixes to local
url determination
add test for usgs.ned.get_tile_urls comparing local to webservice
save file as geotiff rather than generating vrt virtual raster file
If check_modified is False and file exists, do nothing. This enables
stuff to work when local cached version of file is present and internet
is absent
put in a NotImplemented exception for get_tile_url w/ 1/9 arc-second NED
pull common functions into utils.raster, add fix for clipping that
reprojects bbox coordinates to raster projection before clipping