Check parametric surface editor nodes for circular references

[Sunderlandkyl]

Within vtkMRMLParametricSurfaceEditorNode, the input and output node references are checked to ensure that continuous update is disabled if a node is referenced in both the input and output.

This check should be done for all nodes in the scene to ensure that no circular references exist throughout the entire scene.

It may also be enough to add a parameter to a the rule base class to ensure that it won't process an another update if one is already in progress.