code: public static double logPdf(double x, double mu, double sigma)
      {
          return -0.5*log(2*PI) - log(sigma) + -pow(x-mu,2)/(2*sigma*sigma);
      }
docs: Computes the log probability of a given value
  @param x the value to the get log(pdf) of
  @param mu the mean of the distribution
  @param sigma the standard deviation of the distribution
  @return the log probability