Rover: add support for sailboats

[rmackay9]

We should add support for Sailboats to the Rover firmware.

Features of sailboats (guesses from a person with only limited sailing experience):

• at least two main controls: rudder (for heading) and main sail servo (which indirectly provides thrust). It's possible that some main sail controls use ropes while others use a hard bar.
• we probably need a wind vane sensor and thus a new sensor driver added to read in the heading from the wind vane.
• when travelling into the wind, speed is increased by pulling in the main sail but at the expense of the boat's roll angle. When travelling downwind, speed is increased by loosening the main sail (?)
• boats have a maximum angle into the wind that they can achieve.

Some more concrete steps that would be required:

• add a new "Sailboat" option to the FRAME_CLASS parameter (see here)
• in SRV_Channel library's SRV_Channel.h add a new k_main_sail option to the Aux_servo_function_t enum. Users will set the SERVOx_FUNCTION to this number to specify which output control the mainsail
• write a new wind-vane library to pull data from the wind-vane sensor. This library might also include helper functions to say whether the vehicle is pointing upwind or downwind (or maybe this should be in the Rover class in a new sailboat.cpp file)
• enhance the APM_Control/AR_AttitudeControl library to add new get_mainsail_out_speed() and get_mainsail_out_stop() methods. the former should accept a desired speed in m/s and return a main-sail position value as a value from -1 to +1.
• enhance the AP_MotorsUGV class by adding new set_mainsail() and get_mainsail() methods. These will simply handle the conversion of the -1 to +1 value to a servo output.
• modify the Rover's calc_throttle method to call a new calc_mainsail method.

There's probably a lot more to do but hopefully the above is a start. The project steps should probably be:

• first get the boat sailing in Manual mode
• develop the windvane sensor driver library
• use Acro mode to develop and test the speed and steering controls. it may be useful to temporarily modify Acro mode so that either steering or mainsail controls are manual so that the two control can be test/developed separately.
• test AUTO or Guided mode with waypoints placed so the boat can achieve them without tacking
• enhance Rover's calc_steering_to_waypoint() to handle tacking if the direction to the waypoint if too close to the wind.

[lvale]

this one can be tricky....

tacking needs to occur. the algo to calculate an optimal course between points might become a real pain.

Turns and course changes also a real challenge if we don't want ArduSub involved :)

[rmackay9]

This is in master now so closing!