updated the "set number of rovers" in GUI to allow 0 rovers
updated the "set number of rovers" in GUI to allow 0 rovers

In order to facilitate debugging and testing, I'm adding an extra
option to include 0 rovers to the GUI.

BCLab-UNM/SwarmBaseCode-ROS,BCLab-UNM/SwarmBaseCode-ROS,BCLab-UNM/SwarmBaseCode-ROS,BCLab-UNM/SwarmBaseCode-ROS,BCLab-UNM/SwarmBaseCode-ROS,BCLab-UNM/SwarmBaseCode-ROS