Whatsnew for new chunking strategy.

Author: pp-mo

docs/iris/src/whatsnew/contributions_2.3.0/newfeature_2018-Jul-25_dask_chunking.txt

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+* Iris now makes better choices of dask chunk sizes, when loading from netcdf
+  files:  If a file variable has specified, small chunk, Iris will now choose
+  dask chunks which are a multiple of these, up to a default target size.
+  This is particularly relevant to files with an unlimited dimension, which
+  previously could produce a large number of small chunks.
+  In addition, Iris now takes its default chunksize from the default configured
+  in dask itself, i.e. `dask.config.get('array.chunk-size')`.