auto chunks

[d-v-b]

At the moment, if you attempt to call Array.create without specifying chunks or chunk_shape, a ValueError is raised. But I think we should allow automatically creating chunks if the chunks are not specified. In the short term we can use the same heuristic that zarr v2 uses, and in the longer term we could consider making this behavior a configuration point.

[jhamman]

This should be easy to port from v2.

[d-v-b]

this is implemented in #2042

[d-v-b]

closed by #2042