updated the april cube's mass in the model file
updated the april cube's mass in the model file

The mass of the cube in the model.sdf file was not set properly and
has been changed to the correct value.

original mass = 0.1
new mass = 0.01

A side effect of this mass difference was that it caused the simulation
to slow down considerably.

This bug only occurs when creating customized worlds because the prebuilt
worlds used a different file for setting up cubes with the correct mass value.

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