allow switching between confidence scores in reader

[julian-risch]

Let's have only one score in answers but support switching between the two options: probability and score
To this end, the prediction head needs to sort based on a switch here:
https://github.com/deepset-ai/FARM/blob/cf6f96ffcedeb7b1341278dcdd6437e99efaa75b/farm/modeling/prediction_head.py#L1467
Right now, answers contain both probability and score:

{'answer': 'Eddard',
 'context': 's Nymeria after a legendary warrior queen. '
            'She travels with her father, Eddard, to '
            "King's Landing when he is made Hand of the "
            'King. Before she leaves,',
 'document_id': 'ba2a8e87ddd95e380bec55983ee7d55f',
 'meta': {'name': '43_Arya_Stark.txt'},
 'offset_end': 78,
 'offset_end_in_doc': 153,
 'offset_start': 72,
 'offset_start_in_doc': 147,
 'probability': 0.9899835586547852,
 'score': 15.927592277526855}

Related issue in Haystack #1220