Test for pybraincompare atlas

[vsoch]

Need to test if saved atlas object (eg labels, unique values) corresponds to image. It's exactly the same for a non-resampled image, however small regions can be eliminated with resampling even using nearest neighbor approach. The atlas labels come from the xml file, so it could be the case at a much larger voxel dimension, for much smaller regions, that the two lists do not coincide.

A warning should be added to the creation atlas step if a region is eliminated with a resampling.