A minor tweak to allow for the ward linkage in clusterer.apply_agglomerative_clustering

Changes are fine, but can you add NumPy doc style documentation?

Will that work? Or it is too extensive and you prefer to have it as:

metric : str

documentation looks fine this way. Thanks.

Thanks, feature and docs look good to me!

Do we want to add tests and/or an example (basically why and when p would us this)?

There is a flu_timeseries_cluster.py example for the clustering in general. I don't think we need an additional example. The reference to the scipy documentation should suffice. Clusterer itself is 100% tested, but we currently don't explicitly test for the metrics kwarg. I can see arguments for and against adding test for this. For now, I am fine with merging this.

Thanks @mikhailsirenko!