[ENH] ward_tree: return distances

Modified `ward_tree` to return distances for both the structured and unstructured versions,
useful to have when plotting the dendrogram with scipy.
nitpick: `np.asarray` ?

We probably would want to define a temporary array of size (n_nodes - n_samples) and then have a counter to update the inert value, after every merge, but that might be overkill.