Bio.PDB.Entity.Entity; get_list => iter, detach_child => delitem, has_id => contains

Originally reported as "[missing __delitem__ in Bio.PDB.Entity.Entity](https://redmine.open-bio.org/issues/2879)"

Entity.__delitem__ has been implemented: [https://github.com/biopython/biopython/commit/77e0ee396901ad7fb6ab58a2366558956549e582](https://github.com/biopython/biopython/commit/77e0ee396901ad7fb6ab58a2366558956549e582)
so del struct[0] etc. now work as expected.

The only thing left would be the enhancements Eric mentioned:

Eric Talevich wrote:

NB: It looks like Entity supports some methods that are handled just as well by the appropriate magic methods and Python syntax: get_list => iter, detach_child => delitem, has_id => contains. We should eventually deprecate those methods, make some others non-public, and promote the use of properties and magic syntax instead, I think.