Added the operator to frc971 SensorReader.

Took the operator fuction out of the year specific SensorReader, needed
to make a empty RunInteration in order for it to work.

Change-Id: I0ec02ab293c1f8eba56d6365383105e860f69cdd
diff --git a/frc971/wpilib/sensor_reader.h b/frc971/wpilib/sensor_reader.h
index 3abe1b0..b3ced4c 100644
--- a/frc971/wpilib/sensor_reader.h
+++ b/frc971/wpilib/sensor_reader.h
@@ -36,11 +36,17 @@
   // Stops the pwm trigger on the next iteration.
   void Quit() { run_ = false; }
 
+  virtual void RunIteration() = 0;
+
+  void operator()();
+
  protected:
   // Uses the pwm trigger to find the pwm cycle width and offset for that
   // iteration.
   void RunPWMDetecter();
 
+  int32_t my_pid_;
+
   ::std::unique_ptr<frc::DigitalInput> pwm_trigger_;
 
   frc::DigitalGlitchFilter fast_encoder_filter_, medium_encoder_filter_,