Minor styling and remove debug print

src/exdf/data_reduction/method.py

@@ -23,7 +23,6 @@ def is_train_selection(x):
 
 
 def is_index_expression(x):
-    print(x, type(x))
     if isinstance(x, (slice, list)):
         return True
 

src/exdf/write/sd_writer.py

@@ -323,8 +323,7 @@ def build_sources_index(sources):
         else:
             instrument_indices[sd.source] = {
                 grp: sd.data_counts(labelled=False, index_group=grp)
-                for grp in sd.index_groups
-            }
+                for grp in sd.index_groups}
 
     return control_indices, instrument_indices