set assign_torsions to true by default

setting it to false by default was creating too much downstream
trouble. the proper fix for not having to waste any CPU cycles
assigning torsions (which are barely ever looked at) would be to
have GetPhiTorsion/GetPsiTorsion/GetOmegaTorsion return temporary
Dihedral objects with the same interface than TorsionHandle.
These would be created on the fly.