code: def get_run_number():
      
      try:
          f = open(os.path.join(os.getenv('DATA_HOME', "."), 'RUNNUM'), 'r')
          num = int(f.read())
          f.close()
      except Exception:
          num = 1
      f = open(os.path.join(os.getenv('DATA_HOME', "."), 'RUNNUM'), 'w')
      f.write(str(num + 1))
      f.close()
      return num
docs: Get a run number for this execution of the model system, for
      identifying the output hdf5 files).
  
      Returns
      -------
      The integer number for this run of the model system.