Get `ishape_last` for the last slice in init

Common/util_eddies.py

@@ -101,6 +101,10 @@ class SHPC_class:
             for i in range(self.n_slices)
         ]
 
+        # We only need ishape_last for the last slice, for one
+        # orientation, in order to define n_dates:
+        self.get_ishape_last(self.n_slices - 1, self.def_orient)
+
         self._n_shapes = [
             {"Anticyclones": 0, "Cyclones": 0} for i in range(self.n_slices)
         ]