[Pcloud]StimProducer: Impl addAttachedStimBuffIfNotExists

Reduces code duplication, centralizes checking and enforces consistent
behaviour across producers.

Also reordered the writes to the sh_ptr<StimulusBuffer>s such that
the pointers are written last.

include/user/stimulusProducer.h

@@ -76,6 +76,9 @@ public:
 		const std::shared_ptr<device::DeviceAttachmentSpec>
 			&deviceAttachmentSpec) = 0;
 
+	bool addAttachedStimulusBufferIfNotExists(
+		const std::shared_ptr<StimulusBuffer>& buffer);
+
 	virtual void destroyAttachedStimulusBuffer(
 		const std::shared_ptr<StimulusBuffer>& buffer);