Add indents for JSON files for readbility

[calvinchai]

The library writes json files minimized which is difficult to read.
e.g.

{"dtype":"<u1","filters":null,"shape":[348],"order":"C","zarr_format":2,"chunks":[348],"fill_value":null,"compressor":null}

Added indents for readability.

{
    "dtype": "<u1",
    "filters": null,
    "shape": [
        348
    ],
    "order": "C",
    "zarr_format": 2,
    "chunks": [
        348
    ],
    "fill_value": null,
    "compressor": null
}

[asinghvi17]

Should this be a choice / keyword argument instead? Maybe it could propagate down from write or zcreate.

[nhz2]

Yes, minimizing the JSON files saves disk space so adding indents should be optional.

[calvinchai]

Agree. I just revised it and made it a keyword arguments

[asinghvi17]

Looks good to me, thanks for the PR! @meggart should probably review as well.

Test failures are from the pangeo AWS bucket that went down.