add get_decay_chain for custom decay chain

support custom decay chain as

@register_decay_chain("custom_NR")
class MyDecayChain(AmpDecayChain):
    def init_params(self, name=""):
          self.a = self.add_var(name+"a", is_complex=True)

    def get_amp(self, data_c, data_p, **kwargs):
          p_map = {str(i): i for i in self.outs}
          p1 = data_p[p_map["D"]]["p"]
          p2 = data_p[p_map["pi"]]["p"]
          p3 = data_p[p_map["D0"]]["p"]      
          eta = tf.constant([1, -1, -1, -1], dtype=tf.float64)
          l_dot = lambda x, y: tf.reduce_sum(x*y*eta, axis=-1)
          a =  - l_dot(p1, p2) + l_dot(p1, p2+p3)*l_dot(p2+p3, p2)/l_dot(p2+p3, p2+p3)
          zeros = tf.zeros_like(a)
          return self.a() * tf.complex(a, zeros)

for the amplitude

a \hat{g}_{\mu \nu} p_K^{\mu} p_{pi}^{\nu}