Attempt to select the "best" tile from all providers for a given spot

A person could just download all tiles from all providers and switch during use, but for the low-bandwidth user it seems useful to provide a single tileset with whatever is best for each individual tile area

- Check all providers for the highest tile level available
- Check all providers for the most recent tile (don't know if this is possible; I think there is some timestamp data in the metadata for each URL?)
- Use pillow to check for cloudy areas (probably just highest overall RGB pixel value, maybe something more complicated like Hamming distance from an all-white tile, maybe even looking for sections of all white)?