MetaDrive-0.4.2.0

This release includes the following updates:

1. SUMO integration
2. Extract a BaseAgentManager class from the old AgentManager, so we can control more things, i.e. traffic lights in SUMO
3. Make the new doc system
4. Add more content to the doc
5. Fix memory leak when repeatedly calling reset() and close()
6. Use multi-pass rendering for sensors, so we can render the terrain in the camera
7. Include SimplePBR in the source code
8. Add more tests for sensors and install them from different sources
9. Remake the PSSM shadow
10. MeshTerrainSystem
11. ROSbridge
12. InstanceCamera
13. Rename TopDownRenderer. Now it supports semantic top-down renderer