Add z axis to json

[eugenevinitsky]

This has three components:

• Rebuild the json and add z to it
• Add z axis to collision checking (so we don't crash into overhead bridges)
• Add z position of agents to the observation space (so we know what lines we can drive through)

[daphne-cornelisse]

Is this done?

[nadarenator]

For the 2nd and 3rd component, there's partial changes in this pr but largely incomplete.