expose `_pdbx_database_status`

[aozalevsky]

We recently added the deposition date to the validation reports, which requires accessing _pdbx_database_status.recvd_initial_deposition_date. If I'm correct, the only way to do it now is by using semi-hidden (because of the _) system._database_status['recvd_initial_deposition_date'] attribute. Since this table provides important information, I believe it's worth exposing it explicitly.

[benmwebb]

That would be easy enough to expose, assuming that's the only data item you need, or that there's a small number of other fields you would want to access. I would be reluctant to expose all of the fields though, as there's ~50 of them and I generally see only a handful used in PDB-IHM depositions (status_code, deposit_site, process_site, recvd_initial_deposition_date). I can start with those 4 and we can always add more later if needed.

[aozalevsky]

I'm using only recvd_initial_deposition_date right now, so I'm ok with a limited number of fields.