code: public static double kendallsTau(Vector a, Vector b) {
          return kendallsTau(Vectors.asDouble(a), Vectors.asDouble(b));
      }
docs: Computes <a href="http://en.wikipedia.org/wiki/Kendall%27s_tau">Kendall's
  tau</a> of the values in the two vectors.  This method uses tau-b, which
  is suitable for vectors with duplicate values.
  
  @throws IllegalArgumentException when the length of the two vectors are
  not the same.