added possibility of multiple agents on same location to tree observation

examples/flatland_2_0_example.py

 import numpy as np
 
-from flatland.envs.observations import TreeObsForRailEnv, GlobalObsForRailEnv
+from flatland.envs.observations import TreeObsForRailEnv
 from flatland.envs.predictions import ShortestPathPredictorForRailEnv
 from flatland.envs.rail_env import RailEnv
 from flatland.envs.rail_generators import sparse_rail_generator
@@ -39,7 +39,7 @@ env = RailEnv(width=100,
               schedule_generator=sparse_schedule_generator(speed_ration_map),
               number_of_agents=20,
               stochastic_data=stochastic_data,  # Malfunction data generator
-              obs_builder_object=GlobalObsForRailEnv(),
+              obs_builder_object=TreeObservation,
               remove_agents_at_target=True
               )
 

flatland/envs/observations.py

@@ -160,7 +160,12 @@ class TreeObsForRailEnv(ObservationBuilder):
         """
 
         # Update local lookup table for all agents' positions
-        self.location_has_agent = {tuple(agent.position): 1 for agent in self.env.agents}
+        self.location_has_agent = dict()
+        for agent in self.env.agents:
+            if tuple(agent.position) in self.location_has_agent:
+                self.location_has_agent[tuple(agent.position)] = self.location_has_agent[tuple(agent.position)] + 1
+            else:
+                self.location_has_agent[tuple(agent.position)] = 1
         # TODO: Update this to handle number of agents at same location
         self.location_has_agent_direction = {tuple(agent.position): agent.direction for agent in self.env.agents}
         self.location_has_agent_speed = {tuple(agent.position): agent.speed_data['speed'] for agent in self.env.agents}