PI-3637: Allow derived coordinates to be removed

[stephenworsley]

A derived coordinate can now be removed by calling Cube.remove_coord(). Fixes #3637. aggregated_by() now handles derived coordinates. The behaviour of aggregated_by is now that derived coordinates will become regular coordinates if they span the dimension aggregated along.

[pp-mo]

Thanks @stephenworsley .

Unfortunately I think merge was a bit premature : I just realised that it still needs a whatsnew.
Can you maybe address that ?

The effective change to 'aggregated_by' also needs a bit more attention, I think.
As a minimum, I'd suggest a separate whatsnew for that.
However, I'm also still very doubtful whether the 'new' behaviour of 'aggregated_by' is a correct resolution : If we have aggregated all the dependency coordinates, can we not construct a proper derived coord for the result in this case?