filter_polymer does not work with AtomArrayStack though it is documented as such.

[k-chrispens]

I am using biotite.structure.filter_polymer, and it says in the docs it should work with an AtomArrayStack, but it currently doesn't:

>>> from biotite.structure import AtomArray, stack, filter_polymer
>>> import numpy as np
>>> arrays = []
>>> for i in range(2):
...     atom_array = AtomArray(3)
...     base_coords = np.array([[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [0.0, 1.0, 0.0]])
...     atom_array.coord = base_coords + i * 0.1
...     atom_array.set_annotation("chain_id", np.array(["A"] * 3))
...     atom_array.set_annotation("res_id", np.array([1, 2, 3]))
...     atom_array.set_annotation("element", np.array(["C", "C", "C"]))
...     atom_array.set_annotation("b_factor", np.array([20.0, 20.0, 20.0]))
...     atom_array.set_annotation("occupancy", np.array([1.0, 1.0, 1.0]))
...     arrays.append(atom_array)
... 
>>> atom_array_stack = stack(arrays)
>>> filter_polymer(atom_array_stack)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "[...]/python3.12/site-packages/biotite/structure/filter.py", line 380, in filter_polymer
    return np.concatenate(list(bool_idx))
                          ^^^^^^^^^^^^^^
  File "[...]/python3.12/site-packages/biotite/structure/filter.py", line 377, in <lambda>
    lambda a: np.full(len(a), check_pol(atom_array(a)), dtype=bool),
                                        ^^^^^^^^^^^^^
  File "[...]/python3.12/site-packages/biotite/structure/atoms.py", line 1192, in array
    names = sorted(atoms[0]._annot.keys())
                   ^^^^^^^^^^^^^^^
AttributeError: 'numpy.ndarray' object has no attribute '_annot'

Looking a bit deeper it seems that the np.split in filter_atoms is returning a np.ndarray rather than an Atom as expected by the downstream code.

[padix-key]

Hi, sorry for the delay and thanks for the report. #866 will fix the problem.