Generic algorithms + example/test for writing a barrier with ZipSelections

Components + a draft for a barrier with the ZipSelection framework. So far (at least the last time I checked) the Barrier only supports KeyedContainers. The concept of a barrier is explained here

port from previous works on Pr::Selection.

cc @nnolte

• open for input if Barriers (with SOAExtensions) is actually something we want/need in the scope of barriercontrolflow

wrt. previous barrier implementations, a scatterer is not needed:

• A transformer with Zipping::transform would just consume the original "all objects" container and put out a new container that is zip compatible. Scattering doesn't need to happen as the ExportedSelection that goes into the barrier also serve as "readers" (constructor arguments for SelectionView) of the output of the transformer.

See below, this MR requires a patch in Gaudi.