edit test_GCS for debugging
1. fix typo in quad_swarm.h
2. register signal handler in test_GCS.cpp for SIGINT now will send kill command to quads
fixbug:
1. forget to sent ack = 2 to the quadcopter after receiving a positioning info with accuracy < 5, (Ground_Station.cpp, negotiate_ref())
2. only set_quad_state() when received a corresponding ack
TODO:
1. sync the state machine of quad_swarm and the state machine of Swarm class on GCS side.