StimBuff: make start/stop virtual;

We can now have the derived StimBuff class implement its own
start()/stop() preamble and epilogue.

include/user/stimulusBuffer.h

@@ -67,7 +67,7 @@ public:
 	StimulusBuffer& operator=(StimulusBuffer&&) = default;
 
 	// Control methods
-	void start()
+	virtual void start()
 	{
 		std::cout << __func__ << ": Starting stimulus buffer for device "
 			<< deviceAttachmentSpec.deviceSelector << std::endl;
@@ -76,7 +76,7 @@ public:
 		scheduleNextTimeout();
 	}
 
-	void stop();
+	virtual void stop();
 
 protected:
 	// Virtual functions for derived classes to override