Description of proposed changes

For remote datasets, we need to know if it's a grid or an image. Currently, we do below:

kind = "image" if name in {"earth_day", "earth_night"} else "grid"

So, kind defaults to grid but is set to image for two special datasets.

This PR adds the kind attribute to explicitly specify if data is a grid or an image.

Let me know if you're OK with the changes.