One Shot - needs better tar file management for SArray background images

[TobyRoseman]

There are two related issues here:

1 - Each time tc.one_shot_object_detector.create(...) is called the tar file is always extracted, even if those files have already been extracted. The extraction takes a little over 30 seconds. So we shouldn't be doing it if we don't need to do it.

2 - The files are extracted to the user's current directory (in an SArray named one_shot_backgrounds.sarray). We should be extracting these files to somewhere in the Turi Create's data cache (i.e. somewhere under the value return by tc.toolkits._pre_trained_models._get_cache_dir(_type="data")).