code: @Override
      public double membership(double x) {
          if (Double.isNaN(x)) {
              return Double.NaN;
          }
          return height * Math.exp((-(x - mean) * (x - mean))
                  / (2.0 * standardDeviation * standardDeviation));
      }
docs: Computes the membership function evaluated at `x`
  
  @param x
  @return ` h \times \exp(-(x-\mu)^2/(2\sigma^2))`
  
  where `h` is the height of the Term,
  `\mu` is the mean of the Gaussian,
  `\sigma` is the standard deviation of the Gaussian