Support subword and word language models

Closes #199 (closed)

Ref https://redmine.teklia.com/issues/4941

dan/datasets/extract/arkindex.py

@@ -355,6 +355,8 @@ class ArkindexExtractor:
         Convert charset to a LM-compatible charset. Ensure that special LM tokens do not appear in the charset.
         """
         logger.info("Preparing language resources")
+        # Add unknown token to charset
+        self.charset.add(self.unknown_token)
 
         # Build LM tokens
         for token in sorted(list(self.charset)):