PAIR=wohe [RFC=0007](https://github.com/googlecartographer/rfcs/blob/master/text/0007-nav-sat-support.md)
This fixes #212.
This adds an option to subdivide sensor_msgs/LaserScan and sensor_msgs/MultiEchoLaserScan messages into multiple point clouds. For slow spinning laser scanners this allows unwarping using googlecartographer/cartographer#408.