Decide whether cubes are iterable

[DPeterK]

At the moment, cubes report as being iterables, but raise an error if you try to iterate over a cube:

>>> isinstance(cube, Iterable)
True

>>> for c in cube:
...     print c

TypeError: Cube is not iterable

This is confusing and potentially a bit awkward, as it looks like Iris can't make its mind up...

Of course this comes down to the presence of the __iter__ method, whose presence means the Cube class reports as being iterable, while the method itself just raises an error. We could:

• remove the __iter__ method, which is simple to do but unfortunately reverses the problem (Cube reports as not being iterable, while you actually can then iterate over a cube on its leading dimension)
• explicitly support iterating by extending __iter__, which is more complicated and potentially undefined
• ... do something more clever than either of these suggestions
• just ignore it

[pp-mo]

Long long ago...
we explicitly stopped cubes being iterables because it was confusing a lot of users.
See : #86

Of course, we might decide that this now no longer applies.
Anyway, please review pros+cons from the older discussion