Add set to bundles source reference
At the current time, DDA can't have duplicates of OAI-PMH-sources.
This means, that sources had to be filtered while reading all of the metadata.
Prime example of this is econstor, basically reading all of it's data for every import, just to discard most of it.
With this fix, DDA can now read multiple sets of one OAI-PMH-source, resulting in way less time reading unnecessary data.
To keep the sources seperate, we expanded the reference of the OAI-PMH-source with it's set, therefore allowing DDA to have multiple of the same source, but reading different sets and keeping them seperate.