Switch subjects to datastreams

[karkraeg]

• subject vocabularies is still a fixture (https://github.com/inveniosoftware/invenio-rdm-records/blob/master/invenio_rdm_records/fixtures/vocabularies.py)
• There is a bunch of itereators (YamlIterator...) which I assume are like datastream readers
• I guess Schemes (VocabularyEntryWithSchemes) are needed to differentiate between different sources for one specific vocabulary. So for example you'd load into subjects from GND and MeSH and so on.
  • I'm mot sure how this would be ported in terms of datastreams