Specify variable shape / chunking?

[TomAugspurger]

Storage formats like Zarr and NetCDF allow variables to be chunked along one or more dimensions (zarr docs).

Would this extension be an appropriate place to capture that information? Building on #6:

{
  "cube:variables": {
    "prcp": {
      "type": "data",
      "description": "...",
      "extent": [0, null],
      "unit": "mm",
      "dimensions": ["time", "y", "x"],
      // new chunking information below //
      "shape": [365, 1000, 1000],
      "chunks": [1, 100, 100]
    }
}

The expectation is that if chunks is specified, it's an array of integers equal to the number of dimensions. The integers indicate the chunk size. Zarr, at least, doesn't allow non-uniform chunks, so a simple list of integers is used. If non-uniform chunking is desired, you would need a list of list of integers.

This overlaps somewhat with proj:shape, but I'm not really sure how extensions interact with each other.

[TomAugspurger]

Let's continue this discussion in #8.