title: Iterate through folders, then subfolders and print filenames with path to text file
body: <p>I am trying to use python to create the files needed to run some other software in batch.
  For part of this I need to produce a text file that loads the needed data files into the software. 
  My problem is that the files I need to enter into this text file are stored in a set of structured folders. </p>
  
  <p>I need to loop over a set of folders (up to 20), which each could contain up to 3 more folders which contain the files I need. The bottom level of the folders contain a set of files needed for each run of the software. The text file should have the path+name of these files printed line by line, add an instruction line and then move to the next set of files from a folder and so on until all of sub level folders have been checked.</p>