Postprocessor: unify component selection for MeshAdaptor/VTUOutput/Quantities into the `Postprocessor

[tamiko]

This is long overdue: Currently, we are selecting and extracting components by hand separately in each of these classes. We should avoid this wasted effort and code duplication and simply handle everything in the Postprocessor class.

This probably needs to be done with some sort of "subscription" design from the consumer classes so that the postprocessor only computes and extracts values when needed. Another tricky part will be the speed requirements for the Quantities::accumulate() call because it gets called after every time step...