h5py compatibility in v3

[jhamman]

Zarr-Python currently supports most of the h5py API in groups/arrays. As part of #1583, we've decided to break this to clean up the core API going forward.

Going forward, h5py compatibility could be provided in a set of wrapper classes.

Marking this as help-wanted!

[joshmoore]

@clbarnes' https://github.com/clbarnes/h5py_like might be something to look at.

[clbarnes]

Yes, I'd hope that would do most of the heavy lifting! Was written a while ago so might need some updates but we'll see if/ when it gets used.