Fix crash when dz_fields=0

[AngelFP]

There is currently a bug where the code will crash when dz_fields = 0

Traceback (most recent call last):
  File "C:\Users\ferran\Repos\Wake-T\examples\track_plasma_qs2d.py", line 45, in <module>
    bunch_list = plasma.track(bunch, opmd_diag=opmd_diag)
  File "C:\Users\ferran\Repos\Wake-T\wake_t\beamline_elements\field_element.py", line 105, in track
    tracker = Tracker(
  File "C:\Users\ferran\Repos\Wake-T\wake_t\tracking\tracker.py", line 82, in __init__
    field.adjust_dt(self.t_final)
  File "C:\Users\ferran\Repos\Wake-T\wake_t\fields\numerical_field.py", line 74, in adjust_dt
    n_updates = np.ceil(t_final / self.dt_update)
ZeroDivisionError: float division by zero