Fix Ensemble.batch(on=None/str)

Closes #202

src/tape/ensemble.py

@@ -4,6 +4,7 @@
 import warnings
 import json
 import requests
+from collections.abc import Collection
 
 import dask.dataframe as dd
 import numpy as np
@@ -728,6 +729,8 @@ def batch(self, func, *args, meta=None, use_map=True, compute=True, on=None, **k
 
         if on is None:
             on = self._id_col  # Default grouping is by id_col
+        if not isinstance(on, Collection):
+            on = [on]  # Convert to list if only one column is passed
 
         # Handle object columns to group on
         source_cols = list(self._source.columns)