Iris: add tether and payload example

[srmainwaring]

Draft: do not merge this version.

Add a model for a 4m x 6mm rope tether and provide example with tether and payload attached to an Iris.

Tether model originally sourced from https://github.com/RigidWing/sitl_gazebo and modified to correct physical properties (inertia and dynamics).

Usage

Follow the instructions for the standard Iris example in the README.

Details

Tether

• approximates 6 mm diameter Dyneema rope
• radius: 3 mm
• mass per metre: 0.026 kg
• tether length: 4 m
• tether mass: 0.1 kg
• approximated using 20 segments connected by universal joints

Payload

• shape: sphere
• mass 0.75 kg
• radius: 0.05 m

Figure: iris and tether in initial configuration

Figure: iris and tether hovering

Figure: waypoint mission

Other changes

• Update Iris base_link collision to prevent intersection with tether (no self collide, so not strictly required)

[rmackay9]

Wow, fantastic! I was thinking of adding support for reducing oscillation of slung payloads for an upcoming competition later this year.

[tridge]

@srmainwaring thanks!