code: def pdf(self, x, e=0., w=1., a=0.):
          
          t = (x-e) / w
          return 2. / w * stats.norm.pdf(t) * stats.norm.cdf(a*t)
docs: probability density function
          see: https://en.wikipedia.org/wiki/Skew_normal_distribution
          :param x: input value
          :param e:
          :param w:
          :param a:
          :return: