Update um_fields_subset.py

Fix the filename

Author: leoberhelman

src/um_fields_subset.py

@@ -108,7 +108,7 @@ def initialize_output_file(ff):
     file_copy.fields = []
     return file_copy
 
-def create_default_outname(f, suffix="_subset"):
+def create_default_outname(filename, suffix="_subset"):
     """
     Create a default output filename by appending a suffix to the input filename.
     If an output filename already exists, a number will be appended to produce a unique output filename.