Remove argument `i_slice` of `comp_ishape`

Since getting `i_slice` is now cheaper.

Common/util_eddies.py

@@ -221,18 +221,15 @@ class SHPC_class:
         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):
+    def comp_ishape(self, date, eddy_index, orientation):
         """Compute the location in the shapefiles: return `(i_slice, ishape)`.
-        Compute i_slice if i_slice is None.
 
-        Returns None if ishape_last is None. Crashes if i_slice is not
-        None and date is not in i_slice. Returns None if eddy_index is
-        greater than the maximum value.
+        Returns None if ishape_last is None. Returns None if
+        eddy_index is greater than the maximum value.
 
         """
 
-        if i_slice is None:
-            i_slice = self.get_slice(date)
+        i_slice = self.get_slice(date)
 
         ishape_last = self.get_ishape_last(i_slice, orientation)