examples/wolf_sheep: Don't allow dump moves

Agents now move completely random in WolfSheep. That makes no sense whatsoever, so this PR makes them move a little bit less dump.

- Wolf moves to random cell with sheep on them, if available (otherwise completely random)
- Sheep move to a random cell without a wolf (if available), and preferably with grass.

This enables sheep to actually "search" for grass, wolfs to search for sheep and sheep to not move to a cell with wolves.

More importantly, it shows of some nice selection mechanics with the cell space.

mesa/examples/advanced/wolf_sheep/agents.py

@@ -37,7 +37,8 @@ def feed(self):
     def step(self):
         """Execute one step of the animal's behavior."""
         # Move to random neighboring cell
-        self.cell = self.cell.neighborhood.select_random_cell()
+        self.move()
+
         self.energy -= 1
 
         # Try to feed
@@ -62,6 +63,27 @@ def feed(self):
             self.energy += self.energy_from_food
             grass_patch.fully_grown = False
 
+    def move(self):
+        """Move towards a cell where there isn't a wolf, and preferably with grown grass."""
+        cells_without_wolves = self.cell.neighborhood.select(
+            lambda cell: not any(isinstance(obj, Wolf) for obj in cell.agents)
+        )
+        # If all surrounding cells have wolves, stay put
+        if len(cells_without_wolves) == 0:
+            return
+
+        # Among safe cells, prefer those with grown grass
+        cells_with_grass = cells_without_wolves.select(
+            lambda cell: any(
+                isinstance(obj, GrassPatch) and obj.fully_grown for obj in cell.agents
+            )
+        )
+        # Move to a cell with grass if available, otherwise move to any safe cell
+        target_cells = (
+            cells_with_grass if len(cells_with_grass) > 0 else cells_without_wolves
+        )
+        self.cell = target_cells.select_random_cell()
+
 
 class Wolf(Animal):
     """A wolf that walks around, reproduces (asexually) and eats sheep."""
@@ -74,6 +96,16 @@ def feed(self):
             self.energy += self.energy_from_food
             sheep_to_eat.remove()
 
+    def move(self):
+        """Move to a neighboring cell, preferably one with sheep."""
+        cells_with_sheep = self.cell.neighborhood.select(
+            lambda cell: any(isinstance(obj, Sheep) for obj in cell.agents)
+        )
+        target_cells = (
+            cells_with_sheep if len(cells_with_sheep) > 0 else self.cell.neighborhood
+        )
+        self.cell = target_cells.select_random_cell()
+
 
 class GrassPatch(FixedAgent):
     """A patch of grass that grows at a fixed rate and can be eaten by sheep."""