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. If I'm using it here, either:
 * I forgot to change it back, or
 * it needs to use `_name` to avoid any chance of `AttributeError`s.

My suspicion is that the second is the more likely case.