added pub representation

models/publication.py

@@ -5,3 +5,8 @@ class Publication:
         self.doi = doi
         self.preprint_id = preprint_id
         self.other_id = other_id
+
+    def __repr__(self):
+        ids = [("PMC_ID", self.pmc_id), ("PM_ID", self.pm_id), ("DOI", self.doi),
+               ("PREPRINT ID", self.preprint_id), ("OTHER ID", self.other_id)]
+        return ' '.join([str(id) for id in ids if id[1] is not None])