Allow arbitrary initial condition in exit_entry_times

[Datseris]

Recent PR re-worked the return times code: #278 .

Currently, the code is set up so that the trajectory always starts for the set center. However, it would be very nice if we allow any arbitrary initial condition irrespective of cell center. This is easy to achieve. There are comments in the source code of what needs to be done:

    # TODO: Improve the algorithm so that starting within u0 is not mandatory.
    # Useful because `u0` can often be a fixed point.
    # The logic that needs to change is first `transit_return_times` and then
    # to actually check in `exit_entry_times` if we start inside the set,
    # and thus set the `prev_outside` accordingly.