[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.

wpilibcExamples/src/main/cpp/snippets/EncoderDrive/cpp/Robot.cpp

@@ -26,7 +26,7 @@ class Robot : public wpi::TimedRobot {
     rightLeader.AddFollower(rightFollower);
   }
   void AutonomousPeriodic() override {
-    // Drives forward at half speed until the robot has moved 5 feet, then
+    // Drives forward at half velocity until the robot has moved 5 feet, then
     // stops:
     if (m_encoder.GetDistance() < 5) {
       drive.TankDrive(0.5, 0.5);
@@ -43,8 +43,9 @@ class Robot : public wpi::TimedRobot {
   wpi::Spark leftFollower{1};
   wpi::Spark rightLeader{2};
   wpi::Spark rightFollower{3};
-  wpi::DifferentialDrive drive{[&](double output) { leftLeader.Set(output); },
-                               [&](double output) { rightLeader.Set(output); }};
+  wpi::DifferentialDrive drive{
+      [&](double output) { leftLeader.SetDutyCycle(output); },
+      [&](double output) { rightLeader.SetDutyCycle(output); }};
 };
 
 #ifndef RUNNING_WPILIB_TESTS

wpilibcExamples/src/main/cpp/snippets/EncoderHoming/cpp/Robot.cpp

@@ -14,12 +14,12 @@
 class Robot : public wpi::TimedRobot {
  public:
   void AutonomousPeriodic() override {
-    // Runs the motor backwards at half speed until the limit switch is pressed
-    // then turn off the motor and reset the encoder
+    // Runs the motor backwards at half velocity until the limit switch is
+    // pressed then turn off the motor and reset the encoder
     if (!m_limit.Get()) {
-      m_spark.Set(-0.5);
+      m_spark.SetDutyCycle(-0.5);
     } else {
-      m_spark.Set(0);
+      m_spark.SetDutyCycle(0);
       m_encoder.Reset();
     }
   }

wpilibcExamples/src/main/cpp/snippets/LimitSwitch/cpp/Robot.cpp

@@ -6,7 +6,6 @@
 #include "wpi/framework/TimedRobot.hpp"
 #include "wpi/hardware/discrete/DigitalInput.hpp"
 #include "wpi/hardware/motor/PWMVictorSPX.hpp"
-#include "wpi/hardware/rotation/Encoder.hpp"
 
 /**
  * Limit Switch snippets for frc-docs.
@@ -14,24 +13,25 @@
  */
 class Robot : public wpi::TimedRobot {
  public:
-  void TeleopPeriodic() override { SetMotorSpeed(m_joystick.GetRawAxis(2)); }
-  void SetMotorSpeed(double speed) {
-    if (speed > 0) {
+  void TeleopPeriodic() override { SetMotorVelocity(m_joystick.GetRawAxis(2)); }
+  void SetMotorVelocity(double velocity) {
+    if (velocity > 0) {
       if (m_toplimitSwitch.Get()) {
         // We are going up and top limit is tripped so stop
-        m_motor.Set(0);
+        m_motor.SetDutyCycle(0);
       } else {
-        // We are going up but top limit is not tripped so go at commanded speed
-        m_motor.Set(speed);
+        // We are going up but top limit is not tripped so go at commanded
+        // velocity
+        m_motor.SetDutyCycle(velocity);
       }
     } else {
       if (m_bottomlimitSwitch.Get()) {
         // We are going down and bottom limit is tripped so stop
-        m_motor.Set(0);
+        m_motor.SetDutyCycle(0);
       } else {
         // We are going down but bottom limit is not tripped so go at commanded
-        // speed
-        m_motor.Set(speed);
+        // velocity
+        m_motor.SetDutyCycle(velocity);
       }
     }
   }

wpilibcExamples/src/main/cpp/snippets/ProfiledPIDFeedforward/cpp/Robot.cpp

@@ -25,10 +25,11 @@ class Robot : public wpi::TimedRobot {
   void GoToPosition(wpi::units::meter_t goalPosition) {
     auto pidVal = m_controller.Calculate(
         wpi::units::meter_t{m_encoder.GetDistance()}, goalPosition);
-    m_motor.SetVoltage(wpi::units::volt_t{pidVal} +
-                       m_feedforward.Calculate(
-                           m_lastSpeed, m_controller.GetSetpoint().velocity));
-    m_lastSpeed = m_controller.GetSetpoint().velocity;
+    m_motor.SetVoltage(
+        wpi::units::volt_t{pidVal} +
+        m_feedforward.Calculate(m_lastVelocity,
+                                m_controller.GetSetpoint().velocity));
+    m_lastVelocity = m_controller.GetSetpoint().velocity;
   }
 
   void TeleopPeriodic() override {
@@ -44,7 +45,7 @@ class Robot : public wpi::TimedRobot {
   wpi::Encoder m_encoder{0, 1};
   wpi::PWMSparkMax m_motor{0};
 
-  wpi::units::meters_per_second_t m_lastSpeed = 0_mps;
+  wpi::units::meters_per_second_t m_lastVelocity = 0_mps;
 };
 
 #ifndef RUNNING_WPILIB_TESTS