Rawspeed support for the crazy Nikon subformats in the D800 and D810

This adds support for a couple of quirky formats in the D800 and D810:

  * D800 now has the correct whitepoint on 14bit uncompressed images (it was clipping before)
  * sNEF (similar to Canon sRAW) giving a RGB image. Darktable is currently applying a whitebalance to it which is wrong. This should be easy to fix if anyone cares
  * D810 14bit uncompressed images just needed a hint in cameras.xml
  * D810 12bit uncompressed images are now properly detected. Annoyingly there seems to be a bug in the firmware and these are being reported as compressed images. The fix is reasonably sane, just check that the image size is width*height*12bit