Automatic rescaling?

[mfisher87]

We could calculate the minimum and maximum values in a data array and automatically set the rescale range based on that. This would be slow for large datasets, though, but it would only need to happen once. Right now, the rescale parameter accepts either None or a tuple of two numbers. We could also accept the string "auto" (or maybe True?) to trigger automatic rescaling.

For the NDVI example, the data appears dark everywhere because the values are all 0-1, so we pass rescale=(0,1).