code: def fields_checker(fields):
      
      ## make sure fields will work
      if isinstance(fields, int):
          fields = str(fields)
      if isinstance(fields, str):
          if "," in fields:
              fields = [str(i) for i in fields.split(",")]
          else:
              fields = [str(fields)]
      elif isinstance(fields, (tuple, list)):
          fields = [str(i) for i in fields]
      else:
          raise IPyradWarningExit("fields not properly formatted")
  
      ## do not allow zero in fields
      fields = [i for i in fields if i != '0']
  
      return fields
docs: returns a fields argument formatted as a list of strings.
      and doesn't allow zero.