[wpimath] Replace Speeds with Velocities

[calcmogul]

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.

[rzblue]

I think the instances that refer to PWM output should be changed to something else- speed doesn't make sense, but velocity doesn't either.

[calcmogul]

This function in PWMMotorController seems to say it's a pulse time in microseconds whose range is normalized to [-1, 1].

  /**
   * Takes a speed from -1 to 1, and outputs it in the microsecond format.
   *
   * @param speed the speed to output
   */
  protected final void setSpeed(double speed) {
    if (Double.isFinite(speed)) {
      speed = Math.clamp(speed, -1.0, 1.0);
    } else {
      speed = 0.0;
    }

    if (m_simSpeed != null) {
      m_simSpeed.set(speed);
    }

    int rawValue;
    if (speed == 0.0) {
      rawValue = m_centerPwm;
    } else if (speed > 0.0) {
      rawValue = (int) Math.round(speed * getPositiveScaleFactor()) + getMinPositivePwm();
    } else {
      rawValue = (int) Math.round(speed * getNegativeScaleFactor()) + getMaxNegativePwm();
    }

    m_pwm.setPulseTimeMicroseconds(rawValue);
  }

Should it be time-related? I thought of duty cycle, but that doesn't quite make sense because it can be negative.

EDIT: I went with "duty cycle between -1 and 1 (sign indicates direction)".