Don't materialize full or offsets arrays

[tomwhite]

Arrays whose values are a function of the index do not need to be materialized - either in memory or on disk. These include arrays where every entry is the same (created with full), and arrays of offsets (used in map_blocks to provide block IDs to the map function).

Zarr arrays created with full are not expensive to create since they specify write_empty_chunks=False so no chunks are written - just the metadata. However, offsets are materialized to disk: https://github.com/tomwhite/cubed/blob/e21786591d9832a85f2e492641ad2061bdb8c14a/cubed/core/ops.py#L431-L432

Although the array is small since its size is the number of chunks, there is a scalability issue due to the fact that every chunk is stored in a separate file in the Zarr store.

These problems could be solved by writing an array implementation that implements indexing on the fly: either by returning a subarray of fill values for the case of full, or np.ravel_multi_index to turn an index into an offset for an array of offsets.