ENH: helper function fill_cube to fill an iris.cube.Cube

here's a simple wrapper to perform `fill_cube` on an `iris.cube.Cube`.  Any comments are very welcome.
How would you feel about changing this to `inplace` and reversing the meaning? I feel like it would be easier for someone skimming the docstring to realise this is what they want if it were named `inplace` rather than `copy` but it might be a matter of opinion.