Tighten up some parts of the label specification

[dstansby]

While I was implementing this over at ome-zarr-models, and it's in my head, I put together a series of clarifications and improvements to the labels part of the specification. I'm opening this now while it's fresh in my head so I don't forget, but this shouldn't be merged into 0.5, and should wait until #276 is resolved.

Changes:

• Be more clear about what "group" means throughout about by saying "Zarr group" instead of "group"
• Instead of "MUST be an integer data type", pair "MUST" with the explicit list of data types. This makes it clear that e.g., uint128 is not an acceptable data type, where "MUST be an integer data type" could allow that. (I know that in practice the data types are limited by those in the zarr spec, but better to be expclicit than implicit)
• Clarify what is meant by "these MUST NOT contain metadata" when it comes to intermediate groups - I interpreted this as the intermediate groups should not contain OME-Zarr metadata, but I'm not 100% sure what the intent was here?

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