New feature: vertical reduction

If you have a source component providing a field (via yac_put) with a collection size of N, there might be a target component that only wants a single slice of this 3D data (for example atm providing 3D temp field, while ocean only wants surface temp).

Currently, the source component would have two register and provide two fields (a 2D and a 3D one).

@k202160 asked whether it would be possible to have a YAC feature, where the target could select only a single slice of the provded 3D data. This could be done through an option in the XML (potential name: ?vertical reduction?). This would then be extended to other reduction methods like min, max, and avg.

If this feature is to be implemented, one would have to think how this is supposed to interact with the already available time reduction.

@m300083, @m300196 would that be a useful feature in your eyes?