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.
Image::BufferPreload now also perform pre-load if scaling factors are not (0,1)

This is important since the scaling factors are not applied when
retrieving image intensity values, in contrast to the Image::Buffer
class.