Make MultiGrid.place_agent faster (projectmesa#1508)

The condition tested now is a constant time operation when an agent has not been placed in a grid yet, while before it was a linear time operation in relation to the size of the list of agents.

mesa/space.py

@@ -582,10 +582,10 @@ def default_val() -> MultiGridContent:
     def place_agent(self, agent: Agent, pos: Coordinate) -> None:
         """Place the agent at the specified location, and set its pos variable."""
         x, y = pos
-        if agent not in self.grid[x][y]:
+        if agent.pos is None or agent not in self.grid[x][y]:
             self.grid[x][y].append(agent)
-        self.empties.discard(pos)
-        agent.pos = pos
+            agent.pos = pos
+            self.empties.discard(pos)
 
     def remove_agent(self, agent: Agent) -> None:
         """Remove the agent from the given location and set its pos attribute to None."""