Group mobility helper and examples

This MR uses the hierarchical mobility model to provide two example programs showing how group mobility can be configured. One example is in the mobility module and implements what has been called the reference point group mobility model in the literature. The second example is similar but is buildings-aware, and is in the buildings module. A new GroupMobilityHelper is also provided.

Animation of the first example (three nodes moving together): https://www.nsnam.org/~tomh/rpgm-animation.gif

Animation of the second example (buildings-aware group mobility): https://www.nsnam.org/~tomh/outdoor-animation.gif

This is intended to replace !405 (closed).