Cache remote data stores to local on-disk stores

[ruaridhg]

Probably implement this directly in zarr-python

Interesting if also working on local viewers and working with the main package

Store containing 2 stores i.e. local cache and remote and copies from remote store to local cache

Linked to #7

[dstansby]

Someone posted in the Zarr zulip channel with a draft implementation of this here: https://ossci.zulipchat.com/#narrow/channel/423692-Zarr/topic/store.20cache.20example/near/532717302