Merge branch '176-bug-fix-sparse_rail_generator' into 'master'

Resolve "Bug: sparse_rail_generator might not generate an agent/target pair : Results in number of agents == 0."

Closes #176

See merge request flatland/flatland!185

flatland/envs/rail_generators.py

@@ -572,7 +572,7 @@ def sparse_rail_generator(num_cities=5, num_intersections=4, num_trainstations=2
             nodes_per_col = int(np.ceil(nb_nodes / nodes_per_row))
             x_positions = np.linspace(node_radius, height - node_radius, nodes_per_row, dtype=int)
             y_positions = np.linspace(node_radius, width - node_radius, nodes_per_col, dtype=int)
-            city_idx = np.random.choice(np.arange(nb_nodes), num_cities)
+            city_idx = np.random.choice(np.arange(nb_nodes), num_cities, False)
 
             node_positions = _generate_node_positions_grid_mode(city_idx, city_positions, intersection_positions,
                                                                 nb_nodes,
@@ -666,7 +666,7 @@ def sparse_rail_generator(num_cities=5, num_intersections=4, num_trainstations=2
                                     0,
                                     width - 1)
                 tries = 0
-                while (station_x, station_y) in train_stations \
+                while (station_x, station_y) in train_stations[trainstation_node] \
                     or (station_x, station_y) == node_positions[trainstation_node] \
                     or rail_array[(station_x, station_y)] != 0:  # noqa: E125