Improve start gate laser mission

[fnivek]

• Set up the start_gate_laser mission to continuously check the position of the start gate to make sure the goal position is still correct
• Make it go fast!
  • Stop about 1-3 m in front of the first gate
  • Find second gate
  • Place waypoint 1-3 m behind second gate
  • Send full speed command

[fnivek]

• Output a marker before the mission starts so that we know it will go to the right place