Make caching of satellite imagery more transparent

[ejm714]

Add information in docs about caching and/or make more configurable.

CyFi caches downloaded imagery to the /tmp/{feature_config_hash} dir by default. We may want to expose the cache_dir in the CLI or at least surface to users how large the cache ended up being and printing the location. We want to avoid filling up space on a user's computer silently.

We download many more cropped Sentinel-2 images than we end up using (as we calculate cloud area for images within 30 days and then use the most recent image that has fewer than 5% clouds).

Each Sentinel-2 item is approximately 2.5MB since we save 15 bands.