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

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

wpilibc/src/main/native/cpp/PIDController.cpp

@@ -373,6 +373,16 @@ double PIDController::GetError() const {
   }
 }
 
+/**
+ * Returns the current average of the error over the past few iterations.
+ *
+ * You can specify the number of iterations to average with SetToleranceBuffer()
+ * (defaults to 1). This is the same value that is used for OnTarget().
+ *
+ * @return the average error
+ */
+double PIDController::GetAvgError() const { return GetError(); }
+
 /**
  * Sets what type of input the PID controller will use.
  */