Enhance PDB File Import to Support Trajectory Data in Avogadro

[vinayakjeet]

Pull Request #1524

This PR updates pdbformat.cpp to improve the handling of PDB trajectories, aligning with community guidelines for file processing without integrating GUI elements. Key enhancements include:

• Frame Separation with ENDMDL: Implements detection of ENDMDL to identify and separate frames within PDB trajectories, enabling accurate animation and analysis of sequential models.

• Frame Position Storage: Accumulates atomic positions per frame until ENDMDL, then stores these as distinct frames in the molecule, facilitating correct trajectory visualization.

• Final Frame Processing: Ensures the last set of positions is added as a frame, improving robustness in handling diverse PDB files.

Closes #1524.

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