Add/Parse Information about Secondary Structure Elements

Add a new dataframe object as `PandasPdb.df['SSE']`, which contains secondary structure element information. This pandas `DataFrame` would have the same number of rows as the coordinate section DataFrame (`'ATOM'`) and columns "helix" and "sheet" with type "bool."

For ease of use, the SSE dataframe could share the dataframe indices with the `PandasPdb.df['ATOM']` section. It may only get tricky if one ('ATOM' or 'SSE') gets updated & reindex and not the other.

Thus, alternatively, SSE information could be added directly to the 'ATOM' DataFrame, for instance as `bool` columns `'helix'` and `'sheet'`.

In any case, I would suggest to make this feature optional; for instance, by calling a function "`parse_sse`" that uses the information provided in the `.pdb_text`.

Locate the PDB parsing entry point and the construction of df['ATOM']; review how pdb_text records are represented. Use the proposed parse_sse entry point to resolve whether SSE is a separate df['SSE'] or columns on ATOM, preserving boolean helix/sheet values and index behavior. Done means secondary-structure information is available through the chosen API and remains consistent with the coordinate data.

Written by the indexing model from the issue text.