Moved MinMaxFilter to io.scif

This was in an ome.xml package because of the use of a MinMaxStore.
However, there doesn't seem to be any compelling need for it in the code
base, and the filter is needed in io.scif for ImgOpener. Thus it was
moved.

An OME-XML subclass could be added later when needed that provides a
storage mechanism.
ImgSaver now supports saving RGB data

Updated ImgSaver so that it can write RGB data.

The compositeChannelCount of a soure ImgPlus is now treated as the RGB
channel count in SCIFIO. The PlanarAccess object used to retrieve image
planes is assumed to return whole (non-interleaved) planes in ascending
order (e.g. R1, G1, B1, R2, G2, B2, etc...).