Fixed issue #1

 (kNN_SAM_Classifier.label_unclassified_pixels(..., threshold=...) only flags pixels if threshold is exceeded by FIRST spectral neighbour). Replaced random value test image by image containing more realistic spectra. Added tests for kNN_SAM_Classifier to ensure the output makes sense.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>