Fix wrong logical operator

[eungbean]

To test that RGB, DEM and Seg. are there,
the logical operator should be and, nor or.

example:
if there are
476_5767_rgb.jp2, 476_5767_dem.tif, 476_5767_seg.tif files in bielefeld/ folder,
then triplet is formed as below:

group=['476_5767_rgb.jp2', '476_5767_dem.tif', '476_5767_seg.tif']

len(group)=3

and

if len(group) != 3 or len(group) != 4:
    print('missing a file in {}: {}'.format(city.name, [g.name for g in group]))

operation produces always True.
So I fix it as below.

if len(group) != 3 and len(group) != 4:
    print('missing a file in {}: {}'.format(city.name, [g.name for g in group]))