[Dask] merge conflicts

[DPeterK]

Add a subsection detailing what to do if you use dask + Iris for parallel file load and encounter merge conflicts when you try and merge the cubes.

For example:

files = glob.glob(iris.sample_data_path('GloSea4', '*.pp'))
cb = db.from_sequence(files).map(iris.load_cube)
dmc = delayed(lambda cubes: iris.cube.CubeList(cubes).merge_cube())(cb)
dmc.compute()

MergeError                                Traceback (most recent call last)
...
MergeError: failed to merge into a single cube.
  Coordinates in cube.aux_coords (scalar) differ: realization.
  Coordinates in cube.aux_coords (non-scalar) differ: forecast_period.