Switch to Saalfeld's mpicbg version 0.6.1

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fiji-compat: go back to official released ij-legacy

A newer beta of ImageJ2 was released in the meantime; we no longer
need to depend on an intermediate ij-legacy release.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Trainable Weka Segmentation: check input image when applying classifier to display error message if needed.