Opt-in to Iris datum support

Recently, Iris has gained support for datum, which is information about the underlying ellipsoid of the earth used in the coordinate systems of earth science data. While that has the potential to make an already difficult area of concern, namely the treatment of data from different sources and models, more difficult, it also offers a clear path for better comparison of data and compatibility. For now, this feature is opt-in via an iris.FUTURES switch, but activating it already now gives us an opportunity to discover potential issues and also reduces the amount of noise due to user warnings from iris.