Adds EXR format to store depth images in float32

[ducha-aiki]

This PR adds the EXR feature to store depth images (or can be normals, etc) in float32.
It relies on openexr_numpy to manipulate EXR images.

[lhoestq]

Hi ! I'mn wondering if this shouldn't this be an Image() type and decoded as a PIL.Image ?

This would make it easier to integrate with the rest of the HF ecosystem, and you could still get a numpy array using ds = ds.with_format("numpy") which sets all the images to be formatted as numpy arrays