code: def neg_logposterior(self, beta):
          
          post = self.neg_loglik(beta)
          for k in range(0,self.z_no):
              post += -self.latent_variables.z_list[k].prior.logpdf(beta[k])
          return post
docs: Returns negative log posterior
  
          Parameters
          ----------
          beta : np.array
              Contains untransformed starting values for latent variables
  
          alpha : np.array
              State matrix
  
          Returns
          ----------
          - Negative log posterior