Carry scaling factors through copy construction for Image::ConstHeader
Carry scaling factors through copy construction for Image::ConstHeader

Since the Image::Buffer classes derive from ConstHeader, this should
ensure that the scaling factors specified in the original header are
properly accounted for in the output image. This relates to issue #2.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3