PX4 ROS2 msg conformity and explicit topics

- .msg files are PascalCase
 - topics are still either per msg or explicitly listed in TOPICS
 - compatible structs are still generated (eg struct msg_s), but ROS2 style px4::msg::Msg is also available

msg/PositionSetpointTriplet.msg

@@ -0,0 +1,8 @@
+# Global position setpoint triplet in WGS84 coordinates.
+# This are the three next waypoints (or just the next two or one).
+
+uint64 timestamp		# time since system start (microseconds)
+
+px4/PositionSetpoint previous
+px4/PositionSetpoint current
+px4/PositionSetpoint next