[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.

romiVendordep/src/main/java/org/wpilib/romi/RomiMotor.java

@@ -16,7 +16,7 @@ public class RomiMotor extends PWMMotorController {
   /** Common initialization code called by all constructors. */
   protected final void initRomiMotor() {
     m_pwm.setOutputPeriod(PWM.OutputPeriod.k5Ms);
-    setSpeed(0.0);
+    setDutyCycle(0.0);
   }
 
   /**

romiVendordep/src/main/native/cpp/romi/RomiMotor.cpp

@@ -8,5 +8,5 @@ using namespace wpi::romi;
 
 RomiMotor::RomiMotor(int channel) : PWMMotorController("Romi Motor", channel) {
   m_pwm.SetOutputPeriod(PWM::kOutputPeriod_5Ms);
-  SetSpeed(0.0);
+  SetDutyCycle(0.0);
 }