add the splitting of documents in eval retriever

[Rob192]

This is to add the splitting function to our retriever evaluation.

The splitting function for the eval data is a new fixture as it was released recently. there may still be some edges cases. At least I referenced one here : deepset-ai/haystack#751 where the script does not work in some specific case (short sentences and anwser at the very beginning of the context).

However, is likely to improved in a near future and allows us to start evaluating the performances of the full stack with different cuts.

[psorianom]

cool! let's go!