StimBuff: make start/stop virtual;

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

stimBuffApis/livoxGen1/pcloudStimulusBuffer.h

@@ -40,6 +40,10 @@ public:
 	PcloudStimulusBuffer(PcloudStimulusBuffer&&) = default;
 	PcloudStimulusBuffer& operator=(PcloudStimulusBuffer&&) = default;
 
+	// Control methods
+	void start() override;
+	void stop() override;
+
 protected:
 	void stimFrameProductionTimesliceInd() override;