[MRG+1] Fix FeatureAgglomeration docs

The docs are not displayed properly here, http://scikit-learn.org/stable/modules/generated/sklearn.cluster.FeatureAgglomeration.html (I keep running into these)
I think `NotImplementedError` is neater only if we expect to implement it in the future. It doesn't seem to be the case here. I think a concrete class should not provide a method that is never meant to be implemented.