Add loading of YA{2,4,8,12,16} tiff images and saving of YA{8,16}.

I have tested loading and saving only on 8 and 16 bit versions on one or two files and they seemed to work. As far as I understand the tiff specification and the code, it should work with these small changes, except for the fact that the standard demands an ExtraSamples tag and this does not write it or check it, which would be nice, but it is more complicated than this.