remove implicit groups

[d-v-b]

this PR removes support for implicit groups (Zarr groups with no metadata documentation) from the spec. See #291 for the motivation. Besides changing the content of the text, I don't know what else to change, so I'm keeping this a draft for now.

A mergeable version of this PR would include a section explaining what implicit groups are, why they were added previously, and why they are now removed. I am happy to add this if it looks like this PR has legs.

Related discussion: #184

[MSanKeys963]

@d-v-b, is this PR ready for review?

[d-v-b]

@MSanKeys963 yes!

[d-v-b]

how can we move forward with this? I think it's important to make these changes quickly, before we get skew between implementations.

[jhamman]

@zarr-developers/steering-council - this PR has sat without review for 4 months. Can we move it forward please?

[rabernat]

Thanks for the push @jhamman.

[joshmoore]

@d-v-b: do you want to take a look at the conflict?

[d-v-b]

@d-v-b: do you want to take a look at the conflict?

done!

[joshmoore]

Thanks, @d-v-b.

Merging as an agreed upon "adjustment" of v3 needed to a "spec bug" introduced by removing the concept of a root directory without re-evaluating the explicit/implicit concept.

This is a change that all implementors need to be aware of. See the list of "Changes after Provisional Acceptance" for other such items.

cc: @zarr-developers/implementation-council