Add support for robots without DMA.

DMA now won't loop if you don't have a dma device.

Change-Id: I8bbb4a2e44dede031f96974b7bc5c7c86889b985
diff --git a/frc971/wpilib/sensor_reader.h b/frc971/wpilib/sensor_reader.h
index fb25360..dc34e1c 100644
--- a/frc971/wpilib/sensor_reader.h
+++ b/frc971/wpilib/sensor_reader.h
@@ -34,10 +34,12 @@
   // Sets the right drivetrain encoder.
   void set_drivetrain_right_encoder(::std::unique_ptr<frc::Encoder> encoder);
 
-  // Sets the dma.
-  void set_dma(::std::unique_ptr<DMA> dma);
-
+  // Adds a sensor to DMA.
   void AddToDMA(DMASampleHandlerInterface *handler) {
+    if (!dma_synchronizer_) {
+      dma_synchronizer_.reset(
+          new ::frc971::wpilib::DMASynchronizer(std::make_unique<DMA>()));
+    }
     dma_synchronizer_->Add(handler);
   }
 
@@ -48,6 +50,8 @@
   void Quit() { run_ = false; }
 
   virtual void RunIteration() = 0;
+  // Runs the DMA iteration after the synchronizer.  This only gets run if a
+  // sensor has been added to DMA.
   virtual void RunDmaIteration() {}
 
   void operator()();