Adding a function for Q-Score computation

This should be implemented like the [`biopandas.pdb.PandasPDB.rmsd`](https://github.com/rasbt/biopandas/blob/master/biopandas/pdb/pandas_pdb.py#L118) static method.

Start by reading the PandasPDB.rmsd static method in biopandas/pdb/pandas_pdb.py, linked from the issue. Determine the expected Q-Score definition and inputs before choosing the appropriate PDB entry point. Done means a matching static method computes Q-Score and is covered by relevant tests.

Written by the indexing model from the issue text.