ENH: Scalar coord promotion.

This PR adds the capability to add a new axis to a cube, optionally to promote a scalar coordinate to a dimension coordinate.

Context:  Users want to save to NetCDF with a resulting 'time' dimension of length 'UNLIMITED' from a cube with a scalar time coordinate.  Currently, iris makes the outermost dimension UNLIMITED when saving.  Giving the user the ability to promote a scalar coordinate, enables them to have such behaviour.