Complete dask demo

[davidhassell]

Hi, not much to say here, other than I have address the commenting points discussed on Thursday. Also cfdm is no longer a dependency, as I now instantiate things in demo.py as

    # ----------------------------------------------------------------
    # Get the data as a lazy array with active capabilities
    # ----------------------------------------------------------------
    f = NetCDFArray(filename="file.nc", ncvar="q")

    # ----------------------------------------------------------------
    # Get the same data as an in-memory numpy array (with no active
    # capabilities)
    # ----------------------------------------------------------------
    nc = netCDF4.Dataset("file.nc", "r")
    x = nc.variables["q"][...]
    nc.close()

    # ----------------------------------------------------------------
    # Instantiate dask arrays from 'f' and 'x', each with the the same
    # arbitrary distribution of dask chunks.
    # ----------------------------------------------------------------
    dask_chunks = (3, 4)
    df = da.from_array(f, chunks=dask_chunks)
    dx = da.from_array(x, chunks=dask_chunks)

[valeriupredoi]

boss level, @davidhassell 🍺 Did you want to add a test that runs the demo or some unit/integration tests? No probs if not, we can do that later 👍

[davidhassell]

Cheers, @valeriupredoi. Don't mind either way on the tests question.

[bnlawrence]

What happens if the keyword ncvar is not present? Would it be better to pass a NetCDF dataset (already instantiated) in along with a variable name (both required)?

[bnlawrence]

(There would be some consequences. I would work through them, but you will have seen I have a git problem right now.)

[davidhassell]

You'll get an exception with no ncvar or no filename. I don't think that it makes sense to pass in a netCDF.Dataset instance, as on principle we don't want those hanging around as open file handles, but we could make the __init__ args positional only. I'll change that.

[bnlawrence]

I'm happy with this. @valeriupredoi Can you please merge.

[valeriupredoi]

cheers, gents! 🍺