distances between cells and their neighbourhood

[keewis]

The neighbours returned by the neighbour search have the original cell as the first value (from ring=0).

With that, the function:

• converts the cell ids to vectors
• computes the dot and cross products between the vector to the original cell and all others
• calls np.arctan2 to determine the angular great-circle distances, and returns the result

If we pass a dask array, that function is instead applied per chunk.