Fixed palette issues:

 1. no more strange colours in the far background.

  ./mwfractal --min_real=-32 --max_real=32 --min_imag=-20 --max_imag=20 -a --colourizer=4

 2. number of hues specified on command line makes sense viewing the image

  ./mwfractal -a --number_hue=1 --colourizer=2

  ./mwfractal -a --number_hue=16 --colourizer=4

Gave the NEM_Violet_to_Red_Lightness class it's own run() overriding the Colourizer::run(). Now there is no more if colourizer == 2 stuff.