Convenience for converting `Cube`s to other Iris objects

[trexfeathers]

raw_cube_from_cf_var() - introduced in #6318 - outputs Cubes where the original object could not be created during loading. The philosophy is that users can fix loading problems to get the coordinate/whatever that they wanted originally. It should therefore be as easy as possible to convert between types.

We already have the from_metadata() function, so perhaps build on that.

If code proves unrealistic, documentation examples would be a good second option.