v3 spec: no compression?

[davidbrochart]

It looks like an array does not necessarily have a compressor ("An array may be associated with a compressor").
How is it encoded in the array metadata? If it is specified by the absence of a compressor key, the spec should be explicit about it.

[davidbrochart]

Maybe we can have the following in the array metadata:

    "compressor": {
        "codec": "https://purl.org/zarr/spec/codec/binary/1.0",
        "configuration": {}
    },

[jakirkham]

Based on discussion, we decided to simply not have a compressor key if the data is not compressed.

[Carreau]

Closed by #94