Merge branch 'external-dbs' into 'v0.9.9'

Added more names for external_db. #330

See merge request !239

nomad/datamodel/datamodel.py

@@ -424,7 +424,7 @@ class EntryMetadata(metainfo.MSection):
         a_search=Search())
 
     external_db = metainfo.Quantity(
-        type=metainfo.MEnum('EELSDB', 'Materials Project'), categories=[MongoMetadata, UserProvidableMetadata],
+        type=metainfo.MEnum('EELSDB', 'Materials Project', 'AFLOW', 'OQMD'), categories=[MongoMetadata, UserProvidableMetadata],
         description='The repository or external database where the original data resides',
         a_search=Search())