Creates matrix of shape |sentences|×|sentences|.

def _create_matrix(self, sentences, threshold, tf_metrics, idf_metrics):
        """
        Creates matrix of shape |sentences|×|sentences|.
        """
        # create matrix |sentences|×|sentences| filled with zeroes
        sentences_count = len(sentences)
        matrix = numpy.zeros((sentences_count, sentences_count))
        degrees = numpy.zeros((sentences_count, ))

        for row, (sentence1, tf1) in enumerate(zip(sentences, tf_metrics)):
            for col, (sentence2, tf2) in enumerate(zip(sentences, tf_metrics)):
                matrix[row, col] = self.cosine_similarity(sentence1, sentence2, tf1, tf2, idf_metrics)

                if matrix[row, col] > threshold:
                    matrix[row, col] = 1.0
                    degrees[row] += 1
                else:
                    matrix[row, col] = 0

        for row in range(sentences_count):
            for col in range(sentences_count):
                if degrees[row] == 0:
                    degrees[row] = 1

                matrix[row][col] = matrix[row][col] / degrees[row]

        return matrix