diff_steer effector

Provides a simple simulation of a differential steer drive system. It is throttleable, reversible and has a separate braking function. Currently turn rate is limited to 0 to pi radians/sec. Power and braking max limits are configurable.  The exposed interface accepts turn, power and braking settings as percentages for simplicity of operation. A suggested enhancement is to make the max turn rate configurable and an issue has been opened for that later enhancement.

Unit tested for simple forward, braking and turning, as well as a compound drive/turn test. All passed and all produce the expected numerics.