If the city has no ground plane, generate one before rendering

[basicNew]

When a city is created, its ground_plane is set to None, failing to generate an SDF if we forget to set it. I think that in most cases (unless until we incorporate terrains) we will want to generate a plane that encloses the whole city, but at the same time let the builder create a custom one if needed. To address this I think a good approach would be to override to_sdf in City along the lines of:

def to_sdf(self):
if self.ground_plane == None:
    self.ground_plane = self._derive_ground_plane()
super....

where _derive_ground_plane() will compute the bounding box of the city and create a plane that encompasses it.