Define `self._i_slice`

Common/util_eddies.py

@@ -106,6 +106,11 @@ class SHPC_class:
 
         self.n_dates = np.ediff1d(self.d_init, to_end=ishape_last.size)
 
+        self._i_slice = []
+
+        for i_slice, n_dates in enumerate(self.n_dates):
+            self._i_slice.extend(n_dates * [i_slice])
+
         self._n_shapes = [
             {"Anticyclones": 0, "Cyclones": 0} for i in range(self.n_slices)
         ]