Concatenate reporting

I've been sitting on this for a while but a little period of quiet finally gave me time to dust this branch off and get it up as a PR :smile: 

Basically this adds reporting for `iris.CubeList.concatenate()` in the same sort of manner as reporting was agreed on for `iris.CubeList.merge()` (give or take the existing differences between merge and concat).
Yes that could blow up horribly in our faces couldn't it :scream: