text: Title: Issue with file handing rainbow files using wradlib
  Body: https://github.com/ARM-DOE/pyart/blob/master/pyart/aux_io/rainbow_wrl.py#L153
  
  proposed by our friends at MeteoSwiss
  ```
  I propose to solve the issue by passing the file handle, not the file name to wradlib, i.e.:
  with open(filename, 'rb') as fid:
  rbf = read_rainbow(fid, loaddata=True)
  Greetings from Switzerland!
  ```
  