Need to make a transition fo ObjPool<int> to std::vector<int> migration.

As part of the ObjPool deprectation it's required to replace ObjPool with std::vector, since it is not used as an object pool here, but as a sparse set of small ints.

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## Acceptance criteria

- [ ] Characterization test on master pinning current behavior of
`getTemplateAtomAttachmentPoint()` / `getTemplateAtomAttachmentPointById()` /
`getTemplateAtomAttachmentPointsCount()` **before and after** in the `remove()`
`postload`-branch; outcome documented, fixed by tests suits.
- [ ] No `ObjPool` in a `base_molecule.h` persists.
- [ ] Signature of `getTemplateAtomAttachmentPointIdxs` updated.
- [ ] All 8 sites sparse iteration converted;
- [ ] Verified no dangling references to outer-vector elements across `resize()`.
- [ ] `obj_pool.h` - deleted as last step of migration.
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