fixed how a-start works. we can now force it to ignore illegal rail elements....

fixed how a-start works. we can now force it to ignore illegal rail elements. This might be a bit dangerous as you need to fix them later one ( done in current code). Maybe better suggestions here!?

examples/flatland_2_0_example.py

@@ -34,7 +34,7 @@ env = RailEnv(width=50,
               height=50,
               rail_generator=sparse_rail_generator(num_cities=10,  # Number of cities in map (where train stations are)
                                                    num_intersections=0,  # Number of intersections (no start / target)
-                                                   num_trainstations=50,  # Number of possible start/targets on map
+                                                   num_trainstations=100,  # Number of possible start/targets on map
                                                    min_node_dist=10,  # Minimal distance of nodes
                                                    node_radius=4,  # Proximity of stations to city center
                                                    num_neighb=3,  # Number of connections to other cities/intersections