Add path api

Fix

msg/GlobalPathSetpoint.msg

@@ -0,0 +1,16 @@
+# Path setpoint in NED frame
+# Input to Guidance controller
+# Needs to be kinematically consistent and feasible for smooth flight.
+# setting a value to NaN means the state should not be controlled
+
+uint32 MESSAGE_VERSION = 0
+
+uint64 timestamp # time since system start (microseconds)
+
+float64 lat			# Latitude, (degrees)
+float64 lon			# Longitude, (degrees)
+float32 alt			# Altitude AMSL, (meters)
+# NED local world frame
+float32[3] tangent # Unit vector tangent to path
+float height_rate
+float32 curvature