make mosaic and clipping the raster tiles optional
convert to list if single tile is passed to function
make web services primary method of getting metadata, add ability to
retrieve availability of ned data (bounding boxes)
attr 37->LYR_URL causing null returns on get_attribute_download_only call
move rasterio and proj imports into mosaic and clip function. This way
the rest of the raster download tools work even if gdal is not installed