Add Simbad.query_objectids(), a method to return a list of identifiers

from Simbad as an astropy.table.Table.

In the process, split SimbadResult into per-output-type subclasses,
SimbadVOTableResult, SimbadBibcodeResult, and SimbadObjectIDs result.
The aim is to encapsulate result output parsing functionality into
specialized subclasses, and avoid messy if-else statements.
This description needs clarification.  I *think* this means "Query SIMBAD with an object name, and return a table of all other names associated with that object."  Is that interpretation correct?