Fetches the ComplexData from the location indicated from Obs.value_complex, attaches
ComplexData onto the Obs and returns the Obs.
The ComplexData is returned in the format specified by the view (which can be null).
This view is typically a contract between the view and the handler that has been registered,
so they those two know the types of views that can be handled.
obs - an obs without complex data filled in
view - nullable view type. This is defined by the ui and view/handler