code: def logpdf_link(self, link_f, y, Y_metadata=None):
          
          return -link_f + y*np.log(link_f) - special.gammaln(y+1)
docs: Log Likelihood Function given link(f)
  
          .. math::
              \\ln p(y_{i}|\lambda(f_{i})) = -\\lambda(f_{i}) + y_{i}\\log \\lambda(f_{i}) - \\log y_{i}!
  
          :param link_f: latent variables (link(f))
          :type link_f: Nx1 array
          :param y: data
          :type y: Nx1 array
          :param Y_metadata: Y_metadata which is not used in poisson distribution
          :returns: likelihood evaluated for this point
          :rtype: float