fix: set traci default view if it doesn't exist

[KevynKelso]

This change allows you to use LIB_SUMO_AS_TRACI=1 with use_gui=True in some situations.

[LucasAlegre]

Hi @KevynKelso, thanks for this contribution!

Can you explain a bit what this does, and in which cases is it possible to use the GUI with libsumo?

[KevynKelso]

Hey @LucasAlegre thanks for getting back to me!

libtraci added this DEFAULT_VIEW parameter to libtraci.gui at the bottom of their init.py I assume for convenience purposes. However, this parameter does not exist in the libsumo implementation, so when LIBSUMO_AS_TRACI=1, accessing traci.gui.DEFAULT_VIEW throws an AttributeError.

For me, this was the only change required to use both LIBSUMO_AS_TRACI=1 and use_gui=True with the following code snippet:
env = SumoEnvironment( net_file="nets/2way-single-intersection/single-intersection.net.xml", route_file="nets/2way-single-intersection/single-intersection-vhvh.rou.xml", out_csv_name=info_csv, single_agent=True, use_gui=True, num_seconds=1800, render_mode="human", )

I also tested with the petting zoo environment and that seems to work without issue as well, but I can't say it will work in every situation. It does however, seem to make sumo-rl more user friendly in the event someone is trying to use the gui and forgot to unset LIBSUMO_AS_TRACI, or is trying to get better performance with the gui or something like that.

Also I'm using sumo version 1.19.0