Issue with geoviewer.py when loading specific bands

[adehecq]

The argument parser of geoviewer.py is incorrect. For argument "-band" it says: "int, which band to display (start at 0) for multiband images (Default is 0).".
When looking at the code, this argument is passed directly to Raster.show, which has a different behavior:

• the default is that all bands are displayed, not the first band: for 3 band image, it is shown as RGB (this could be a problem for > 4 band!).
• if set, the band indexing actually starts at 1.
  The second point also means that if setting "-band 0", the last band is actually displayed, which is confusing!

I suggest the following:

• update the argument parser of geoviewer.py to be correct
• possibly, add a condition in Raster.show so that negative values raise an error