SimilarityModel3D and RigidModel3D only fit rotations, not reflections

[aschampion]

Title says it all, with more discussion here: catmaid/CATMAID#1857

My questions:

1. I'd be happy to fix and PR this here, also, but not knowing how/where this library is used, the proper rotation restriction may be preferred. Should I bother with a PR?
2. If "yes" to 1: There's a lot of functionally identical but slightly syntactically varying code duplicated between AffineModel3D/SimilarityModel3D/RigidModel3D. It'd be easier for me to clean some of this up as part of a PR, but your preference may be for minimal change.