Add missing method in seaice_drift.py (#4161)

Author: sloosvel

esmvaltool/diag_scripts/seaice_drift/seaice_drift.py

@@ -631,6 +631,9 @@ def dependencies(self):
         """Return a dict mapping from constructor names to coordinates."""
         return {"lat": self.lat, "lon": self.lon}
 
+    def _calculate_array(self):
+        pass
+
     def _derive(self, lat, lon):
         def in_polygon(lat, lon):
             """Check if point is inside polygon."""