Rover: add support for mars rover style steering

[rmackay9]

We should add support for the steering system used by the Mars rover. Here is a discussion on the Rover forums about this.

[srmainwaring]

@rmackay9 - if you're happy with a Lua script I've previously posted something that will do this on discourse. It would benefit from an update, options to set parameters based on frame dimensions, etc.

It's also a nice model to have about if you're looking for examples to demo ROS integration, there's a ROS nav stack that goes along with it.

[srmainwaring]

There is an example Gazebo model and script available in SITL/models: https://github.com/ArduPilot/SITL_Models/pull/95. The script has variables customised to the dimensions of the Sawppy rover. The following list outlines the tasks to turn it into a standalone applet for libraries/AP_Scripting/applets:

• [ ] Add params for wheel coordinates / base dimensions.
• [ ] Add params for servo allocation.
• [ ] Add a slew rate / or stop when switching from standard to pivot style steering (at the point when the steering reverses as the instantaneous centre of curvature moves inside the wheel base radius).