Bugfix/dropped commands

[hello-jackson]

This fix addresses a bug that occurs when the joint_trajectory_server interprets a FollowJointTrajectoryGoal where all trajectory points are already within the acceptable_joint_error threshold and then within 1/80th of a second attempts to interpret another FollowJointTrajectoryGoal. The second call of execute_cb() calls the robot.push_command() method at a frequency higher then 80 hz resulting in a dropped motor command. This dropped motor command causes the joint_trajectory_server to find it's goals unreachable until the default_goal_timeout_duration of 10.0 seconds is reached.

In this fix the init_execution() method of each command group now returns a boolean value indicating if a subsequent call to robot.push_command() is appropriate. If at least one of commanded joints returns true (and thusly it can be inferred that joint will take time to move) robot.push_command() is called. The command groups controlling Dynamixel servos will always return false if active because they never require a push_command() call.

This bug can be recreated on the dev/noetic branch with this gist but not on the bugfix/dropped-commands branch.