[wpimath] Replace Speeds with Velocities (#8479)

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.

robotpyExamples/UnitTest/subsystems/intake.py

@@ -26,9 +26,9 @@ class Intake:
         self.piston.set(wpilib.DoubleSolenoid.Value.kReverse)
         self.motor.set(0)  # turn off the motor
 
-    def activate(self, speed: float) -> None:
+    def activate(self, velocity: float) -> None:
         if self.isDeployed():
-            self.motor.set(speed)
+            self.motor.set(velocity)
         else:  # if piston isn't open, do nothing
             self.motor.set(0)