compiler: Avoid premature expansion of avg just like derivs

Author: FabioLuporini

devito/types/basic.py

@@ -1032,13 +1032,13 @@ def _evaluate(self, **kwargs):
         for d, i in self._grid_map.items():
             retval = retval.diff(d, deriv_order=0, fd_order=2, x0={d: i})
 
-        # Evaluate. Since we used `self.function` it will be on the grid when evaluate
-        # is called again within FD
+        # Evaluate. Since we used `self.function` it will be on the grid when
+        # evaluate is called again within FD
         if self._avg_mode == 'harmonic':
             from devito.finite_differences.differentiable import SafeInv
             retval = SafeInv(retval.evaluate, self.function)
         else:
-            retval = retval.evaluate
+            retval = retval._evaluate(**kwargs)
 
         return retval