Ignore temporary files written by vi

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
PGM format: only add channel axis for color images

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Revert "PGM format: only add channel axis for color images"

For the moment, vigra-imglib2 handles only XYC images (to accomodate
SCIFIO without the reverted patch). We will eventually need to be able to
create both 2D and 3D images with vigra-imglib2's ImgFactory; when that
happend, this revert can be reverted.

This reverts commit 62d965fdee30d006e4d822b6e38d879785509145.