spec v3: progressive encoding

[davidbrochart]

I'm wondering if progressive encoding could be supported in Zarr. It is a technique often used on the web, where a low resolution image can be downloaded and displayed first, and then refined as the download continues (see e.g. https://cloudinary.com/blog/progressive_jpegs_and_green_martians). Zarr currently supports only full resolution contiguous chunks, so if you want to have a global view of the data, even if you are going to coarsen it afterwards, you have to first get all the data. Progressive encoding would allow to save a lot of bandwidth in this case, which is particularly useful for e.g. visualization. But I'm not sure if it would be easy to fit into the current architecture, or if there is interest in it.