Add winch plugin

[srmainwaring]

Add a winch plugin that can be used to control a tow-cable for launching aircraft.

• Set cable length.
• Set cable mass per unit length.
• Attach cable to tethered body.
• Set location of the winch.
• Control cable length.
• Control winch speed.
• Auto stop winch at cable length set points (min, max).

Track progress on branch: https://github.com/srmainwaring/asv_sim/tree/wips/wip-winch-plugin