Issue with `_find_separatrix()` for CDB loaded data

Issue with _find_separatrix() in versions:

• branch development, i.e. ref 87d071eb
• branch unstable, i.e. a414debc

There are no such issues in an older code, ref 85877fd2, which is installed in modules as pleque/unstable.

Code to reproduce

from pleque.io.compass import cdb as pleque_io_cdb

# Any of these produce an error
eq = pleque_io_cdb(21673, 1150.0, revision=-1)
# eq = pleque_io_cdb(21554, 1210.0, revision=-1)
# eq = pleque_io_cdb(20908, 1150.0, revision=-1)
# eq = pleque_io_cdb(15964, 1160.0, revision=-1)

eq.plot_overview()

Trace

/compass/home/hecko/repos/pleque/pleque/io/tools.py:35: UserWarning: rename 'Rt' to 'R' does not create an index anymore. Try using swap_dims instead or use set_index after rename to create an indexed coordinate.
  ds = ds.rename({'Rt': 'R', 'Zt': 'Z'})
/compass/home/hecko/repos/pleque/pleque/io/tools.py:35: UserWarning: rename 'Zt' to 'Z' does not create an index anymore. Try using swap_dims instead or use set_index after rename to create an indexed coordinate.
  ds = ds.rename({'Rt': 'R', 'Zt': 'Z'})

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[12], line 7
      2 # eq = pleque_io_cdb(21673, 1150.0, revision=-1)
      3 # eq = pleque_io_cdb(21554, 1210.0, revision=-1)
      4 # eq = pleque_io_cdb(20908, 1150.0, revision=-1)
      5 eq = pleque_io_cdb(15964, 1160.0, revision=-1)
----> 7 eq.plot_overview()

File ~/repos/pleque/pleque/core/equilibrium.py:986, in Equilibrium.plot_overview(self, ax, **kwargs)
    981 def plot_overview(self, ax=None, **kwargs):
    982     """
    983     Simple routine for plot of plasma overview
    984     :return:
    985     """
--> 986     return self._plot_overview(ax, **kwargs)

File ~/repos/pleque/pleque/core/equilibrium.py:995, in Equilibrium._plot_overview(self, ax, **kwargs)
    993 from pleque.utils.plotting import plot_equilibrium
    994 # plt.figure()
--> 995 return plot_equilibrium(self, ax=ax, **kwargs)

File ~/repos/pleque/pleque/utils/plotting.py:93, in plot_equilibrium(eq, ax, colorbar, **kwargs)
     90 ax.fill_between(eq._first_wall[:, 0], eq._first_wall[:, 1], color='lightgrey')
     91 ax.plot(eq._first_wall[:, 0], eq._first_wall[:, 1], color='k', lw=2,
     92         label='First wall')
---> 93 sep = eq.separatrix
     94 in_fw = eq.in_first_wall(sep)
     95 ax.plot(sep.R[in_fw], sep.Z[in_fw], color='C3', lw=2, alpha=0.5)

File ~/repos/pleque/pleque/core/equilibrium.py:1322, in Equilibrium.separatrix(self)
   1320 if not self._limiter_plasma:
   1321     if not hasattr(self, '_separatrix'):
-> 1322         self._find_separatrix()
   1323     return self._as_fluxsurface(self._separatrix)
   1324 else:

File ~/repos/pleque/pleque/core/equilibrium.py:1346, in Equilibrium._find_separatrix(self)
   1343 for j in separatrix:
   1344     # todo: this is not separatrix... for example in limiter plasma and without first wall
   1345     intersection = np.array(self.first_wall._string.intersection(j._string))
-> 1346     if len(intersection) > 0:
   1347         self._separatrix = j.as_array(("R", "Z"))
   1348         found = True

TypeError: len() of unsized object