Support querying other NED tables

Hi, I've read the docs and astroquery.ned.get_table() can take one of the following as a table argument: ‘photometry’|’positions’|’diameters’|’redshifts’|’references’|’object_notes’.

Is it possible to query the other tables available in ned? in particular the 'Classifications' table?

Start with the astroquery.ned.get_table() entry point and its handling of the listed table arguments. Check how the NED Classifications table is exposed, then make querying it consistent with the existing supported tables; done means classifications can be requested successfully and the behavior is covered by the project's relevant checks.

Written by the indexing model from the issue text.