PodioOutput: add ability to promote subset to full collections

[Zehvogel]

Sometimes we would like to drop the original collection and only write out one of its subset collections, for this we would need to "promote" the former subset collection to be a real collection. We could either try to automatically determine when this happens or have the user supply a dedicated list of subset collections that should be promoted as in Marlin/LCIO.

[tmadlener]

This will most likely require a bit of work also in podio because

• subset collections are currently effectively stored in the same way as relations, i.e. we only store the object id for each element in a subset collection and then resolve this again when reading back. This simply needs a "bit of work" when storing a subset collection, to instead store itself as if it were a proper collection, mostly by simply cloning the necessary elements.
• However, there is also a conceptual problem here, because a subset collection is effectively a collection of handles. Hence, if some object has a relation to an element of a subset collection, that relation is not aware that there is a subset collection in play and it will only know about the "original" object in the full collection that the subset collection points into. I am not sure if there is a general way of handling this.

[Zehvogel]

This will most likely require a bit of work also in podio

I feared that this will be the case, I also opened a ticket there now :)

AIDASoft/podio#655