Add a chunk cache to zarr-python 3

[ruaridhg]

This is something that would be very helpful to have ported from zarr-python 2: https://zarr.readthedocs.io/en/v2.18.5/api/storage.html#zarr.storage.LRUStoreCache

Find code in version 2 branch of repo

[ruaridhg]

Read data from hard-drive, but cache in RAM