Add function for structural alignments via least-squares fit

A `staticmethod` least-squares superposition. We could call it `align` and implement it with similar parameters as the [rmsd](https://github.com/rasbt/biopandas/blob/master/biopandas/pdb/pandas_pdb.py#L118) function. In addition, it would be nice to have a `substructure` parameter for substructure alignment -- maybe accepting an iterable of residue numbers here.

Start with the rmsd function in biopandas/pdb/pandas_pdb.py and compare its parameters with the proposed align interface. Define how least-squares superposition and residue-number substructure selection should behave, then verify that the resulting alignment handles the requested full-structure and substructure cases.

Written by the indexing model from the issue text.