Added PIDController::GetAvgError() back in (#749)

It got removed during the LinearDigitalFilter change (#38) instead of deprecated
by mistake.

wpilibj/src/main/java/edu/wpi/first/wpilibj/PIDController.java

@@ -515,6 +515,19 @@ public class PIDController implements PIDInterface, LiveWindowSendable, Controll
     return getContinuousError(getSetpoint() - m_pidInput.pidGet());
   }
 
+  /**
+   * Returns the current difference of the error over the past few iterations. You can specify the
+   * number of iterations to average with setToleranceBuffer() (defaults to 1). getAvgError() is
+   * used for the onTarget() function.
+   *
+   * @deprecated Use getError(), which is now already filtered.
+   * @return     the current average of the error
+   */
+  @Deprecated
+  public synchronized double getAvgError() {
+    return getError();
+  }
+
   /**
    * Sets what type of input the PID controller will use.
    *