[feat] added get_model and get_models fct to mmcif

[kierandidi]

Code of Conduct

Description

Added get_model and get_models methods to PandasMmcif class, similar to PandasMmtf.

Related issues or pull requests

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./biopandas/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under biopandas/docs/sources/ (if applicable)
• Ran PYTHONPATH='.' pytest ./biopandas -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./biopandas/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./biopandas

[a-r-j]

LGTM! Can you add a couple tests? https://github.com/BioPandas/biopandas/blob/main/biopandas/pdb/tests/test_multiple_models.py