Load chunking control

[pp-mo]

Added a rather crude chunking control
- by dimension only, (compare with the altogether more sophisticated facility in Iris)

I'm adding this mostly because it enables a simple workaround to fix
this current problem arising

I think one could argue that this is a shortcoming in Dask, but on re-reading the Dask docs, I'm satisfied that it is really no intention of Dask ever to provide a magic automatic interface which handles cases like this.

So, I'm sure they would say "just give us chunk sizes that make sense", and indeed the error message arising when you attempt to pass chunks="auto" in that case is simply
ValueError: auto-chunking with dtype.itemsize == 0 is not supported, please pass in ``chunks`` explicitly.

[pp-mo]

Possibly warrants a "how-to" addition also ?