Quick pass on initial top level metadata

[abkfenris]

Cribbed from https://github.com/carbonplan/ndpyramid/blob/935878f35f585fa42db3e50c9f130636fc74996d/ndpyramid/regrid.py#L229-L237

.zmetadata:

{
  "metadata": {
    ".zattrs": {
      "multiscales": [
        {
          "datasets": [
            {
              "path": "0"
            },
            {
              "path": "1"
            },
            {
              "path": "2"
            },
            {
              "path": "3"
            },
            {
              "path": "4"
            },
            {
              "path": "5"
            }
          ],
          "type": "reduce",
          "metadata": {
            "method": "pyramid_reproject",
            "version": "0.0.5",
            "args": [],
            "kwargs": {
              "levels": 6,
              "pixels_per_tile": 128
            }
          }
        }
      ]
    },
    ".zgroup": {
      "zarr_format": 2
    }
  }
}