Add payload release mode in plane

[shovan777]

Feature request

Is your feature request related to a problem? Please describe.
_A Payload dropped with do_set_servo() simply opens the servo at the given location so, we have no control over where the payload drops. The problem is much more grave in fixed wing UAVs as payload is dropped from a moving aircraft making it accelerate and drop in uncontrolled manner.

Describe the solution you'd like
_A Make a mode called payload_release which will plan the location to release the payload and move the aircraft in such a way that payload is dropped to the targeted location precisely.
Describe alternatives you've considered
_A Other solution may involve beacon system or the whole flight planning and release point selection may be done at a higher level of abstraction in the ground control station.
Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ x ] Plane
[ ] Rover
[ ] Submarine

Additional context
Add any other context or screenshots about the feature request here.

[aabs7]

The code should be written in a standard way and in the right place for adding features in ArduPlane.

[shovan777]

The mode add feature diverges in Plane compared to copter in following ways:

1. land, loiter, like mini modes are completely separate from auto mode in plane
2. there are only start and verify commands and no run command
3. the start and verify command live in command_logic.cpp instead of mode_auto.cpp

[aabs7]

Although the method of adding flight mode varies in ArduPlane compared to ArduCopter, a standard way must be researched and followed in the entire process of adding features. I will work on that and follow up.

[aabs7]

Commit of a1324af above addresses following changes:

• Commented //added & end with //add finish above and below lines of code that are added
• In parameters.cpp, 24:PAYLOADRELEASE added under FLTMODE(1~6). This is the best practice.
• In plane.h mode_payloadrelease object is returned instead of mode_auto object

[aabs7]

Commit of cb8275e above addresses the following changes:

• The bool is_payload_released is removed from plane.h auto_state structure, because this boolean is not auto mode mission state, and is added to ModePayloadRelease flight mode state inside mode.h.
• Special condition checking of MAV_CMD_NAV_PAYLOAD_RELEASE inside mode_auto.cpp is removed because this command can be called and implemented in start_command and verify_command functions inside command_logic.cpp.

Further addition of features:

Addition of features should be done in the update function inside mode_payloadrelease.cpp and verify_payload_release & do_payload_release functions inside command_logic.cpp.

[shovan777]

#2 while implementing the wind based payload drop calculations the faulty wind estimate was observed.

[aabs7]

Up to now

• The plane calculates the release point taking into account wind velocity for the payload to reach target drop point. The calculation of the release point can be found here.
• Intermediate point is calculated from release point parallel to direction plane is flying.
• The plane loiters around center perpendicular from the line of intermediate point and the release point with distance R from the intermediate point and radius R.
• The plane leaves from loitering making tangent towards release point.
  
  In the figure above, waypoint 9 is the target drop point.

To do

• Add servo functionality when reached release point.
• code refactoring.

[aabs7]

The current version supports a single release point calculation. For continuous release point updates, the code has to be extended for the plane to reach changing waypoint.