Support writing multiple `AtomArray` to structure file

[dnlbauer]

The pdb specification allows pdb files with multiple different models. biotite can read these files, but there is no way to write them.

[padix-key]

You are right, this is definitely something Biotite should support. This involves not only the PDB format but all formats supported by Biotite. However, solving this issue requires quite a bunch of work, so it may take some time until I get to fix this.

[ncoish]

Is this issue still a problem? When I try to write an AtomArrayStack to a pdb file I correctly get all models written to the file.

[padix-key]

This issue is about writing different models in terms of different atoms per model (e.g. an additional atom), not just different coordinates. In other words the problem is that a list of AtomArray cannot be given to set_structure(), although the format specification would allow it.

[padix-key]

The issue title is a bit misleading, so I will clarify it.

[ncoish]

This issue is about writing different models in terms of different atoms per model (e.g. an additional atom), not just different coordinates. In other words the problem is that a list of AtomArray cannot be given to set_structure(), although the format specification would allow it.

Understood. Thanks for the clarification!