Deprecate imglib2-io project

This project is no longer developed, and all relevant code has moved
into SCIFIO (io.scif:scifio) at https://github.com/scifio/scifio.

SCIFIO depends on ImgLib2, using its data structures as appropriate in
its core API. The io.scif.img package now provides the ImgOpener and
ImgSaver classes, which have been improved from those of imglib2-io.