Make it easy to extract coords as separate cubes, and the reverse "injection"

[pp-mo]

Motivating examples:

• "get the altitude as an altitude cube"
• "add the data-quality array as an aux-coord".

Actually, this should almost certainly apply to other multidimensional cube components, not just coords.
I.E. cell-measures + ancillaries as well.
( cf. #3600, which proposes similar extensions to iris.util.new_axis )

See reference code ideas in : pp-mo#61

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This idea was originally pushed as a WIP PR : #3485 , but that was confusing.
So, now it's an issue instead.

[stephenworsley]

This is still of interest and we believe there is enough demand to justify doing some work on this.