Feature: add multi sensor subscription

[TSC21]

This adds the possibility to the gazebo_mavlink_interface to subscribe dynamically to multiple sensor sources connected to a model. I started with the lidar sensors, but this can be extended to multiple other sources (like GPS's, multiple camera sources, others).

To do:

• Extend to sonars and other sensors;
• Add a model example with an array of multiple distance sensors.

This feature potentiates the development of onboard sense-and-avoid algorithms. Some changes were also required on the simulator module in PX4. PR following upstream.

@dagar @jkflying @Jaeyoung-Lim @mrivi @baumanta FYI.

[TSC21]

@mrivi I just extended the iris_obs_avoid model with 4 lidars (one downward, one forward, one left and one right facing). Here's an example output of listener distance_sensor:

pxh> listener distance_sensor

TOPIC: distance_sensor 4 instances

Instance 0:
 distance_sensor_s
	timestamp: 5856000  (0.048000 seconds ago)
	min_distance: 0.2000
	max_distance: 15.0000
	current_distance: 0.2000
	variance: 0.0000
	h_fov: 0.0524
	v_fov: 0.0524
	q: [0.7071, -0.0000, -0.7071, -0.0000]
	signal_quality: -1
	type: 0
	id: 0
	orientation: 25

Instance 1:
 distance_sensor_s
	timestamp: 5856000  (0.048000 seconds ago)
	min_distance: 0.2000
	max_distance: 15.0000
	current_distance: 0.2200
	variance: 0.0000
	h_fov: 0.0524
	v_fov: 0.0524
	q: [-1.0000, 0.0000, -0.0000, -0.0000]
	signal_quality: -1
	type: 0
	id: 1
	orientation: 0

Instance 2:
 distance_sensor_s
	timestamp: 5856000  (0.048000 seconds ago)
	min_distance: 0.2000
	max_distance: 15.0000
	current_distance: 0.2300
	variance: 0.0000
	h_fov: 0.0524
	v_fov: 0.0524
	q: [0.5000, 0.5000, -0.5000, 0.5000]
	signal_quality: -1
	type: 0
	id: 2
	orientation: 2

Instance 3:
 distance_sensor_s
	timestamp: 5860000  (0.044000 seconds ago)
	min_distance: 0.2000
	max_distance: 15.0000
	current_distance: 0.2100
	variance: 0.0000
	h_fov: 0.0524
	v_fov: 0.0524
	q: [0.5000, -0.5000, -0.5000, -0.5000]
	signal_quality: -1
	type: 0
	id: 3
	orientation: 6

[julianoes]

Looks correct from what I can tell. Only one small comment.