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 file_existence_check(self, args):
      input_args = []
      for fname in args:
          if isinstance(fname, str):
              input_args.append(fname)
          # unpack variable string
          if isinstance(fname, list) and all(
              isinstance(s, str) for s in fname
          ):
              input_args += [f for f in fname]
      for path in input_args:
          if len(glob(path)) == 0:
              raise OSError("File not found: " + path)
  
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 9 for calling the IOIterator obj's underlying member method code with example usage: obj.file_existence_check(args) without return types