Demosaic algos

Hi,

I've added igv demosiac from rawtherapee to darktable. It out performs at high noise levels amaze and ppg.  Yielding better resolution on the Sony NEX-C3 and other cameras. At low noise levels zipper-artifacts may occur. I've added an hidden example demosaic code called stagger but which is invisible from the gui. 

At some point it will be replaced by better code.