Writing multichannel datasets + data/metadata interaction

[bogovicj]

Metadata formats may specify or be opinionated about how certain dimensions are stored [1].

Handling this now (Sept 2024) is really up to the programmer, since writing is handled at a low level (blocks). Having an additional higher-level that knows about dimension semantics would help remove some of this burden from developers.

[1] N5-viewer specifies that channels be stored in groups named `c#, OME-Zarr specifies that multiple channel be stored in the same array