用Burg算法估计AR模型参数，进而实现功率谱估计.

形参说明：

x——双精度实型一维数组，长度为n，存放随机序列。

n--整型变量,随机序列的长度。

p--整型变量,AR模型的阶数。

a--双精度实型一维数组，长度为(p十1)。存放AR模型的系数a(0),a(1),...,a(p)。

v--双精度实型指针,它指向预测误差功率，即AR模型激励白噪声的方差。

-with Burg algorithm estimates AR model parameters, thereby realizing the power spectrum estimation. - Participation : x -- double-precision real one-dimensional arrays, length n, storage random sequence. N -- integer variables, random sequence length. P -- integer variables, AR model of order. A -- double-precision real one-dimensional arrays, length (p 10). AR storage coefficient of a model (0), a (1 ),..., a (p). V -- double precision real-pointer, it forecast errors at power, AR model that inspired the white noise variance.