[swarm] add formation control for rotorcraft (#2246)

* [swarm] add formation control for rotorcraft

Based on Ewoud Smeur (INDI) and Hector Garcia de Marina (Formation
control) work. See
https://blog.paparazziuav.org/2017/12/02/pilot-a-super-rotorcraft
- several UAV can be control from the ground (using a joystick for
  instance) by sending proper acceleration setpoint to the INDI guidance
  controller
- configuration of the formation is done from a JSON file (2 example
  files provided)

Some extra changes:
- update accel from IMU in GPS passthrough INS
- add accel setpoint to ABI messages
- by to accel setpoint in INDI guidance
- send ground reference from natnet2ivy
- possibility to have a joystick labeled 'GCS' with input2ivy

conf/modules/fc_rotor.xml

@@ -0,0 +1,24 @@
+<!DOCTYPE module SYSTEM "module.dtd">
+
+<module name="fc_rotor" dir="fc_rotor">
+  <doc>
+    <description>
+      Algorithm for the formation control of a team of rotorcrafts.
+      The vehicles are are running the INDI control and the desired acceleration are given by script from ground.
+    </description>
+  </doc>
+
+  <header>
+    <file name="fc_rotor.h"/>
+  </header>
+
+  <init fun = "fc_rotor_init()"/>
+
+  <datalink message="DESIRED_SETPOINT" fun="fc_read_msg()"/>
+
+  <makefile firmware="rotorcraft">
+    <define name="FC_ROTOR"/>
+    <file name="fc_rotor.c"/>
+  </makefile>
+
+</module>