[Feature Request] Add requested update rate to OpenIGTLinkIF

[jamesobutler]

It would be nice to have the ability to set the rate at which Slicer nodes get updated from incoming OpenIGTlink events. Changes probably needed in here:

SlicerOpenIGTLink/OpenIGTLinkIF/MRML/vtkMRMLIGTLConnectorNode.cxx

Lines 512 to 518 in fc7da7c

	#endif
	volumeNode->SetAndObserveImageData(image);
	volumeNode->SetName(deviceName.c_str());
	this->External->GetScene()->SaveStateForUndo();
	volumeNode->SetDescription("Received by OpenIGTLink");
	vtkDebugWithObjectMacro(this->External, "Name vol node " << volumeNode->GetClassName());
	this->External->GetScene()->AddNode(volumeNode);

My specific use case is being able to acquire >100 FPS using a PLUS device which can save all of its data and then render the image data in Slicer at some slower update rate. If the RequestedRate is lower than the number of events entering, then update the Slicer node only at that rate or if RequestedRate is higher than the number of events, update the Slicer node for all device events. It would be similar to functionality of vtkPlusVirtualCapture which can choose to save image data at some requested frame rate which might be lower than the device source. I figured this feature could be in this repo instead of in vtkPlusOpenIGTLinkServer so that any server sending OpenIGTLink messages to Slicer could benefit from this new requested update rate.

Sometimes I want to only update a live stream in Slicer at maybe 15FPS and save my CPU overhead for other tasks instead of rendering images in Slicer that are unnecessary.