SITL: add simulated slung payload

[rmackay9]

This adds SITL support for a slung payload and is implemented similarly to how the Ship landing ship was implemented.

The payload can be enabled by setting a new SIM_SLUP_ENABLE parameter to 1. In addition the payload's weight, line length and air resistance (aka drag) can also be adjusted through new parameters

This Physics Classroom wiki page was used as a reference to come up with this method for updating the payload and vehicle's movements:

1. update the payload's position, velocity using the previous iterations acceleration
2. check that the payload does not fall below the terrain
3. check if the line is taught and that the payload does not move more than the line length from the vehicle
4. calculate gravity and drag forces on the payload
5. calculate the tension force between the payload and vehicle including force countering gravity, drag and centripetal force
6. update the payload's acceleration using the sum of the above forces
7. update the vehicle's acceleration using the opposite of the above forces

This has been tested in SITL and seems to move in a plausible way.

To add confidence I've checked that the payload reaches the correct maximum speed according to the above linked pendulum document. So with drag removed (e.g. SIM_SLUP_DRAG=0) and the vehicle suddenly stopped so as to cause the pendulum to swing out to 38deg the pendulum reached a maximum speed of 11m/s which is very close to the theoretical 11.17m/s.

PeterB's concerns addressed, txs!