BUG: spatial: Raise a ValueError in sokalsneath if both vectors are entirely false (ticket #876). Also fixed the double appearance of the factor 2 in the docstrings for sokalsneath and sokalmichener.
BUG: spatial: Raise a ValueError in sokalsneath if both vectors are entirely false (ticket #876). Also fixed the double appearance of the factor 2 in the docstrings for sokalsneath and sokalmichener.

datamade/hcluster,datamade/hcluster,datamade/hcluster