Dask intro improvement and review

Here are some improvements and explanation on Dask.

I also fixed some code that wouldn't work near the end (either too heavy on memory, either wrong variable used).

I'd like also to discuss some points:

Warning on spilling, I think this is too complex to introduce yet.
dask.optimize(): should we really introduce this? This makes a different in number of tasks displayed, but upon execution, Dask is clever enough to not execute the useless tasks, I tested with and without optimization on graph, and the result is almost the same. I'm under the impression that Dask does optimize things anyway upon calling compute.

Fixes #50.

pangeo-data / foss4g-2022