The model building process:
1. build the .con .edg .nod files and .netccfg files
2. In powershell, use the command 
"netconvert --node-files cross.nod.xml --edge-files cross.edg.xml --connection-files cross.con.xml -o cross.net.xml"
to create .net file. Notice that in that file, if we specify a traffic light in .nod file, SUMO will autonomously generate
one.
3. build the .add file, specify the traffic light we want (should use different program ID) and the detectors, etc.
4. build the .sumocfg by combing the .net, .add and .rou (could generate by python or mannually).
5. run the .sumocfg file by sumo-gui