require at least one affiliation for contact

metadata_tools/resources/elu-core.json

@@ -59,6 +59,8 @@
           "affiliations": {
             "description": "Names of the affiliated institutions.",
             "type": "array",
+            "minItems": 1,
+            "uniqueItems": true,
             "items": {
               "type": "string"
             }