Atlas name could be improved for anistropic atlases

[adamltyson]

At this line, the atlas name uses the first resolution value: https://github.com/brainglobe/bg-atlasgen/blob/f944baaf69004aca270c29dee50592369969db72/bg_atlasgen/wrapup.py#L111

This isn't particularly useful for non-isotropic atlases, but I'm not sure what would be better.