Remove pointers from children

[douweschulte]

After some thought I think that the pointers from children to its parent are suboptimal. And completely removing them would solve some issues I foresee in the future (eg .remove() on an element while iterating...). So I guess I should vote in favour of removing them altogether, but at the same time I like the idea of having acces to some information, like the backbone() function. But that could be solved by saving a boolean in the Atom. And I guess that function (and some like that I would like to provide) are much less important then the safety and stability of the library.

So removing all pointers and the functions around it would be my proposal, but @TianyiShi2001 I would like some input if you have some? Do you think those pointers serve a purpose or are they just forced by me as I wanted to copy CCTBX?

[TianyiShi2001]

I've always been in favour of removing the pointers because 1) I haven't yet encountered any task that absolutely requires a pointer to the parent (because a reference to the parent is usually implicitly available when accessing the child, for example in a nested for loop), 2) using raw pointers to implement doubly linked trees doesn't look idiomatic (usually this is implemented with Rc<RefCell<T>> and Weak<RefCell<T>> in Rust, and indeed in cctbx this is also implemented with the corresponding counterparts, shared_ptr and weak_ptr)

[douweschulte]

Okay then that is solved. The pointers can be removed. Thanks!