analysis: groupby defaults to included "run", but could be None
When applying transformations, it seems the default groupby is `['run', 'subject']`.

However, this is can be a problem because some runs dont have a run number (if there is only one run, it seems to be optional under the BIDS spec). In this case, a pandas `KeyError` will occur. 
diff --git a/bids/analysis/tests/test_transformations.py b/bids/analysis/tests/test_transformations.py
index a08d5d6d..cad35f84 100644
--- a/bids/analysis/tests/test_transformations.py
+++ b/bids/analysis/tests/test_transformations.py
@@ -39,7 +39,7 @@ def test_product(collection):
 
 def test_scale(collection):
     transform.scale(collection, variables=['RT', 'parametric gain'],
-                    output=['RT_Z', 'gain_Z'])
+                    output=['RT_Z', 'gain_Z'], groupby=['run', 'subject'])
     ents = collection['RT'].index
     groupby = pd.core.groupby._get_grouper(ents, ['run', 'subject'])[0]
     z1 = collection['RT_Z'].values
@@ -79,7 +79,7 @@ def test_orthogonalize_sparse(collection):
     pg_pre = collection['parametric gain'].values
     rt = collection['RT'].values
     transform.orthogonalize(collection, variables='parametric gain',
-                            other='RT')
+                            other='RT', groupby=['run', 'subject'])
     pg_post = collection['parametric gain'].values
     vals = np.c_[rt.values, pg_pre.values, pg_post.values]
     df = pd.DataFrame(vals, columns=['rt', 'pre', 'post'])
diff --git a/bids/analysis/transformations/base.py b/bids/analysis/transformations/base.py
index 58d739e6..ca2965f4 100644
--- a/bids/analysis/transformations/base.py
+++ b/bids/analysis/transformations/base.py
@@ -91,7 +91,7 @@ class Transformation(object):
         >= 3.3, we can't override both new and init. '''
         self.collection = collection
         self.variables = listify(variables)
-        self.groupby = kwargs.pop('groupby', ['run', 'subject'])
+        self.groupby = kwargs.pop('groupby', None)
         self.output = listify(kwargs.pop('output', None))
         self.output_prefix = kwargs.pop('output_prefix', None)
         self.output_suffix = kwargs.pop('output_suffix', None)
['bids/analysis/tests/test_transformations.py', 'bids/analysis/transformations/base.py']