Write semantic segmentation node

[wheitman]

Input: PointCloud2 messages from front and rear lidars Output: Single PointCloud2 with field for point class. Each point has associated class ID, e.g. 0 for vehicle, 1 for pedestrian, 2 for ground

Constraints:

• Should publish at rate >= 1 Hz
• Should detect and classify at least the following: Vehicle, Pedestrian, Drivable Surface (e.g. road, parking lot. Can be combined or separated, it doesn't matter).