Originally started with just swerve, but expanded to diff and mecanum
(docs only) for parity across the drivetrains. Return value checks are
applied when possible to make migration easier and to error loudly if
people forget.
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Co-authored-by: Joseph Eng <91924258+KangarooKoala@users.noreply.github.com>
I left "free speed" alone since that's the technical term for it. In
general, velocity is a vector quantity, and speed is a magnitude (i.e.,
a strictly positive value).
This PR also replaces the speed verbiage in MotorController with duty
cycle.
Fixes#8423.
