validation function checking that root mesh and scaled annotations image roughly match

[alessandrofelder]

This consists of calculating the axis-aligned bounding box (AABB) of the labels in annotation.tiff and of the root mesh, scaling the AABB of the labels and checking that the two are closer than a threshold - the threshold could depend on the pixel size. Just needs to enable us to catch things like brainglobe/brainglobe-atlasapi#213 .