PIDController now supports composition (#976)

For example, an outer position feedback controller can now drive the
reference of an inner velocity feedback controller.

wpilibc/src/main/native/cpp/PIDBase.cpp

@@ -528,6 +528,18 @@ void PIDBase::Reset() {
   m_result = 0;
 }
 
+/**
+ * Passes the output directly to SetSetpoint().
+ *
+ * PIDControllers can be nested by passing a PIDController as another
+ * PIDController's output. In that case, the output of the parent controller
+ * becomes the input (i.e., the reference) of the child.
+ *
+ * It is the caller's responsibility to put the data into a valid form for
+ * SetSetpoint().
+ */
+void PIDBase::PIDWrite(double output) { SetSetpoint(output); }
+
 void PIDBase::InitSendable(SendableBuilder& builder) {
   builder.SetSmartDashboardType("PIDBase");
   builder.SetSafeState([=]() { Reset(); });

wpilibc/src/main/native/include/PIDBase.h

@@ -16,14 +16,13 @@
 #include "Base.h"
 #include "Filters/LinearDigitalFilter.h"
 #include "PIDInterface.h"
+#include "PIDOutput.h"
 #include "PIDSource.h"
 #include "SmartDashboard/SendableBase.h"
 #include "Timer.h"
 
 namespace frc {
 
-class PIDOutput;
-
 /**
  * Class implements a PID Control Loop.
  *
@@ -34,7 +33,7 @@ class PIDOutput;
  * in the integral and derivative calculations. Therefore, the sample rate
  * affects the controller's behavior for a given set of PID constants.
  */
-class PIDBase : public SendableBase, public PIDInterface {
+class PIDBase : public SendableBase, public PIDInterface, public PIDOutput {
  public:
   PIDBase(double p, double i, double d, PIDSource& source, PIDOutput& output);
   PIDBase(double p, double i, double d, double f, PIDSource& source,
@@ -83,6 +82,8 @@ class PIDBase : public SendableBase, public PIDInterface {
 
   void Reset() override;
 
+  void PIDWrite(double output) override;
+
   void InitSendable(SendableBuilder& builder) override;
 
  protected: