Flag retracted entries regularly

[LukasWallrich]

We need to flag retracted articles consistently - both originals and replications. This could either be a continuous integration that runs this (e.g.) once a week and adjust the references in the dataset - or a function that checks RetractionWatch every time the data is loaded. The latter is easier & more accurate, but not ideal for speed and CrossRef load. @LukasRoeseler what would you prefer?

Original retracted? → no problem, just mark them (exp & ann)
Retracted replications → include but flag? (ann, not exp)

[LukasRoeseler]

I prefer the once-a-week solution. Specifically, we could create additional variables for the references (retracted_o and retracted_r).