Feature request

[bogomyag]

Is it possible to implement initial conditions from twiss table at the exit?
It would be nice and consistent if xt.START and xt.END are allowed in
tw.get_twiss_init(at_element=xt.END)

File xtrack/twiss.py

    def get_twiss_init(self, at_element):

        assert self.values_at == 'entry', 'Not yet implemented for exit'

        if isinstance(at_element, str):
            at_element = np.where(self.name == at_element)[0][0]