Overhaul of datalink serializers

[kimakan]

The Datacite/Dublincore serializers for the datalink must include fields like

• contributors
• sizes
• etc.

It has to be compatible with the oai renderers.
The serializers in question:

daiquiri/daiquiri/datalink/serializers.py

Line 8 in 33094c5

class DublincoreSerializer(serializers.Serializer):

daiquiri/daiquiri/datalink/serializers.py

Line 19 in 33094c5

class DataciteSerializer(serializers.Serializer):

Furthermore if possible, it would be nice to simplify other serializers, like
https://github.com/django-daiquiri/daiquiri/blob/33094c5389e0acb0b30fcc1db54b7bb6bafaf0c5/daiquiri/metadata/serializers/datacite.py
Maybe it's not necessary to list all fields in the serializers but only the ones that need a default value. However, it has to be investigated further.