Make `dimension_names` mandatory at array level

[vincentsarago]

dimension_names is an optional attribute in the Zarr V3 spec (https://github.com/zarr-developers/zarr-specs/blob/main/docs/v3/core/index.rst#dimension_names) but spatial:dimensions refers directly to it in the spatial convention.

The dimension names must match the dimension names defined in the array's shape.

Having the spatial:dimensions without Array.dimension_names is kinda useless so I believe the convention should enforce the presenceof dimension_names attribute at array level

[maxrjones]

I agree, this is an oversight that should be corrected. The change here is independent from #23; it would be good to break #25 into two PRs if you're willing @emmanuelmathot.

[emmanuelmathot]

@maxrjones I restricted #25 to the resolution of this issue. Please now let's find a solution in #23 before starting a new PR