Add function that uses os.path.join to create new paths

[JohannesWiesner]

Would be nice to have a function that is similar to get_new_filepath but automatically uses os.path.join behind the scenes? Or maybe obsolete because we can also use get_new_filepath?

[JohannesWiesner]

Example:

Create a new column subject_dir by joining a fixed variable root_dir with each subject's subject id and session name:

df_sourcedata['subject_dir'] = df_sourcedata.apply(lambda row: os.path.join(root_dir,row['subject_id'],row['session']),axis=1)