ImgSaver: use SCIFIOImgPlus to preserve metadata

When saving data, we were always losing format-specific metadata. By
switching to SCIFIOImgPlus signatures, we can use the getMetadata()
method to retrieve format-specific metadata and then use the
Metadata,ImageMetadata translation signature (ImageMetadata still should
come from the ImgPlus in case it has been modified).

However, this is still limited by the fact that any operation that
creates a new ImgPlus will lose the metadata attached to the
SCIFIOImgPlus.
SCIFIOImgPlus: preserve metadata on copy

Updated the copy method of SCIFIOImgPlus to preserve the format-specific
metadata.