class_docstr: Create output filenames that work nicely with multiple input files from
  multiple directories (processing multiple subjects with one command)
  
  Use information from input files, out_dir and out_fnames to generate
  correct outputs which can come from long lists of multiple or single
  inputs.
code: def set_output_keys(self, *args):
      self.out_keys = list(args)
  
imports: import inspect
  import itertools
  import numpy
  import os
  from glob import glob
  from dipy.workflows.base import get_args_default
label_desc: Use image node_id 5 for calling the IOIterator obj's underlying member method code with example usage: obj.set_output_keys() without return types