[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/SimpleDifferentialDriveSimulation/robot.py

@@ -18,7 +18,7 @@ class MyRobot(wpilib.TimedRobot):
 
         # Slew rate limiters to make joystick inputs more gentle; 1/3 sec from 0
         # to 1.
-        self.speedLimiter = wpimath.SlewRateLimiter(3)
+        self.velocityLimiter = wpimath.SlewRateLimiter(3)
         self.rotLimiter = wpimath.SlewRateLimiter(3)
 
         self.drive = Drivetrain()
@@ -43,15 +43,15 @@ class MyRobot(wpilib.TimedRobot):
     def autonomousPeriodic(self) -> None:
         elapsed = self.timer.get()
         reference = self.trajectory.sample(elapsed)
-        speeds = self.feedback.calculate(self.drive.getPose(), reference)
-        self.drive.drive(speeds.vx, speeds.omega)
+        velocities = self.feedback.calculate(self.drive.getPose(), reference)
+        self.drive.drive(velocities.vx, velocities.omega)
 
     def teleopPeriodic(self) -> None:
-        # Get the x speed. We are inverting this because Xbox controllers return
+        # Get the x velocity. We are inverting this because Xbox controllers return
         # negative values when we push forward.
-        xSpeed = (
-            -self.speedLimiter.calculate(self.controller.getLeftY())
-            * Drivetrain.kMaxSpeed
+        xVelocity = (
+            -self.velocityLimiter.calculate(self.controller.getLeftY())
+            * Drivetrain.kMaxVelocity
         )
 
         # Get the rate of angular rotation. We are inverting this because we want a
@@ -60,9 +60,9 @@ class MyRobot(wpilib.TimedRobot):
         # the right by default.
         rot = (
             -self.rotLimiter.calculate(self.controller.getRightX())
-            * Drivetrain.kMaxAngularSpeed
+            * Drivetrain.kMaxAngularVelocity
         )
-        self.drive.drive(xSpeed, rot)
+        self.drive.drive(xVelocity, rot)
 
     def simulationPeriodic(self) -> None:
         self.drive.simulationPeriodic()