Update gps_guidance.ino

Rewrote code using PWMServo.h to solve servo jitter caused by interrupt(1) conflict between Servo.h and SoftwareSerial.h libraries.

The same button can now be used to set the LZ and open the launch gate.  The user can now open the launch gate for 2 seconds to insert the lifting wire loop, which secures glider to kite/balloon.

Optimized by only moving the servo if course has changed, and reducing severity of turns.