Bugfix in pod, and cleanup.

- A correct determination of which sensor was generating each prt has been
  implemented, allowing the data to miss scanlines. It is based on the
  scanline numbers provided in the data
- The pod data is also cleaned up before after reading.
- The code has been cleaned up a little, to follow python standards.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>