code: def variation_of_information(self, other):
          
          (entropy_1, entropy_2, mut_inf) = entropies(self, other)
  
          return entropy_1 + entropy_2 - 2 * mut_inf
docs: calculates Variation of Information Metric between two clusterings
          of the same data - SEE Meila, M. (2007). Comparing clusterings: an
          information based distance. Journal of Multivariate Analysis, 98(5),
          873-895. doi:10.1016/j.jmva.2006.11.013