Avoid downloading all rasters

[lewfish]

If you have many large scenes, the rasters will all be downloaded at once and you will run out of disk space. This is because the download occurs in the constructor of RasterSources. Instead, dwe should download rasters https://github.com/azavea/raster-vision/blob/develop/rastervision/data/raster_source/rasterio_source.py#L79 in the _activate method https://github.com/azavea/raster-vision/blob/develop/rastervision/data/raster_source/rasterio_source.py#L100 and delete them in the _deactivate method.