msg: Add vertical derivative to vehicle position messages

This enables control loops that are sensitive to vertical velocity offsets to use a vertical velocity that is kinematically consistent with the vertical position.

msg/vehicle_global_position.msg

@@ -15,6 +15,7 @@ uint8 alt_reset_counter 		# Counter for reset events on altitude
 float32 vel_n			# North velocity in NED earth-fixed frame, (metres/sec)
 float32 vel_e			# East velocity in NED earth-fixed frame, (metres/sec)
 float32 vel_d			# Down velocity in NED earth-fixed frame, (metres/sec)
+float32 pos_d_deriv		# Down position time derivative in NED earth-fixed frame, (metres/sec)
 float32 yaw 			# Euler yaw angle relative to NED earth-fixed frame, -PI..+PI, (radians)
 float32 eph			# Standard deviation of horizontal position error, (metres)
 float32 epv			# Standard deviation of vertical position error, (metres)

msg/vehicle_local_position.msg

@@ -26,6 +26,7 @@ float32 delta_z
 float32 vx 				# North velocity in NED earth-fixed frame, (metres/sec)
 float32 vy				# East velocity in NED earth-fixed frame, (metres/sec)
 float32 vz				# Down velocity in NED earth-fixed frame, (metres/sec)
+float32 z_deriv				# Down position time derivative in NED earth-fixed frame, (metres/sec)
 
 # Velocity reset delta
 float32[2] delta_vxy