Move concatenate operation from 'iris.experimental' into a CubeList method.
Merge pull request #783 from pp-mo/promote_concat

Moved concatenate from 'iris.experimental' to 'iris'.
Merge branch 'master' of https://github.com/SciTools/iris into float64_pp_save
Merge branch 'float64_pp_save' of ssh://github.com/niallrobinson/iris into float64_pp_save