examples: Introduce acoustic-elastic coupling example

Author: EdCaunt

devito/finite_differences/differentiable.py

@@ -64,6 +64,7 @@ def time_order(self):
     @cached_property
     def grid(self):
         grids = {getattr(i, 'grid', None) for i in self._args_diff} - {None}
+        grids = {g.root for g in grids}
         if len(grids) > 1:
             warning("Expression contains multiple grids, returning first found")
         try:

devito/types/basic.py

@@ -709,6 +709,7 @@ def grid(self):
 
     def _infer_dims(self):
         grids = {getattr(c, 'grid', None) for c in self.flat()} - {None}
+        grids = {g.root for g in grids}
         dimensions = {d for c in self.flat()
                       for d in getattr(c, 'dimensions', ())} - {None}
         # If none of the components are devito objects, returns a sympy Matrix