Add function to parse secondary structure elements from PDBx

[padix-key]

As MMTF is now deprecated (#525) another method is desired to parse the secondary structure from PDB. PDBx files contain annotations of helices and sheets in the struct_conf and struct_sheet_range categories respectively.

Hence, a pdbx.get_sse() function could be implemented.

[ceziegler]

Is there a preferred format for the output?

[padix-key]

I think a dictionary mapping the chain ID to the secondary structure elements (SSE) would be good. I think the SSE should be an ndarray

• either using letters like in https://www.biotite-python.org/apidoc/biotite.structure.annotate_sse.html
• or IntEnum values of a new IntEnum class, as e.g in https://www.biotite-python.org/apidoc/biotite.structure.BondType.html

Residues without defined secondary structure could get a "c" or some COIL value from the enum value assigned

[padix-key]

Thanks by the way for tackling this feature! I assigned the issue to you.

[padix-key]

Hi, are you still planning to implement this feature? If not, this is also fine, but I would free the issue up again.

[ceziegler]

Hi yes, I'll finish this today!