function sumCol(matrix) {
        var sum = Matrix.zeros(1, matrix.columns);
        for (var i = 0; i < matrix.rows; ++i) {
            for (var j = 0; j < matrix.columns; ++j) {
                sum[0][j] += matrix[i][j];
            }
        }
        return sum;
    }
@private
Retrieves the sum at each column of the given matrix.
@param {Matrix} matrix
@return {Matrix}