[dask] task graphs one to many

[DPeterK]

Dask task graphs seem to be optimised for many to one style processing: you load multiple files and perform some sort of aggregation / reduction to arrive at a single result. This does not seem to be the paradigm anything like as often in Iris, where you might need to...

• take multiple statistics of a single cube and return all of them (e.g. retrieve mean and standard deviation concurrently)
• extract multiple sub-cubes out of one or more input cubes
• and so on.

Put together an example of making a graph that looks like this and then computing the graph to easily retrieve the requested data.