Hi,

Thanks for the great package.

In the old version of biopython, as illustrated in https://biopython.org/wiki/GSOC2010_Joao, the Structure class has a build_bilogical_unit() method, which is a convenient API to apply all symmetry operation listed in PDB REMARK 350 section. But I cannot find it in the new version.

I'm wondering is this function moved to elsewhere or it's not implemented for some reason?

Feel free to close this issue, because it's not a bug report.

Motivation

Entity is the parent class of Structure, Model, Chain, and Residue classes, used hundreds of times when building a PDB object. It should be the first target to be visited when refactoring the PDB object creation framework.

Proposed Changes

• Check if any ABC can be used as a base class for Entity, to provide most methods automatically.
• Test the inclusion of slots to reduce memory. Compensate loss of 'flexibility' with xtra dictionary?
• Remove all set/get methods that do not perform any special operations besides setting/getting an attribute.
• Test the replacement of child_list/child_dict by a single exposed children attribute that is based on a dictionary (maybe ordered dictionary? Or sort upon iter).