Use `_i_slice` instead of bisection to get slice

Common/util_eddies.py

 import math
 from os import path
 import sys
-import bisect
 import os
 
 import shapefile
@@ -219,9 +218,8 @@ class SHPC_class:
         return ishape_last
 
     def get_slice(self, date_index):
-        i_slice = bisect.bisect(self.d_init, date_index) - 1
-        assert i_slice >= 0
-        return i_slice
+        assert date_index >= self.d_init[0]
+        return self._i_slice[date_index - self.d_init[0]]
 
     def comp_ishape(self, date, eddy_index, orientation, i_slice=None):
         """Compute the location in the shapefiles: return `(i_slice, ishape)`.