fixes and cleanups to bounding box as well as autoslab interface and implementation

- changed gfx::GfxObj.GetBoundingBox to take boolean argument, allowing obj transform
to be applied; in C++, the default is use_tf=False, in Python, the argument must be
provided
- deprecated Scene::Autoslab(bool,bool) and Scene::AutoslabMax()
- added Scene::Autoslab() to replace above deprecated methods
- added Scene::[SG]etAutoslabMode(int) to set behavior of autoslab (default is fast mode, as before, so no behavior change)
- added Scene::GetBoundingBox(bool use_tf)
- added GetBoundingBox() methods, auto_autoslab, autoslab_mode and bounding_box properties to gfx::Scene python export
- modified gfx::GfxObj::ProcessLimits behavior to take obj transform into acount
- moved some code duplication to mol::Transform::Apply(AlignedCuboid)
- do_autoslab upon adding a new gfx obj is only applied when auto_autoslab is true
- tweaked some in-source documentation
- added str(self) defs to some geom exports