Vizier: refactor to use the generic AstroResponse/AstroRequest and
related caching tools.  Local tests pass now.  Remote tests will be run
when internets r availablz
add a verification step in the data retrieval process to avoid spamming
the ESO servers with invalid queries
eso: simpler return statement as per Derdon's suggestion
eso: fix function name
eso: change 'data_retrieval' to 'retrieve_data'
eso: docs data_retrieval -> retrieve datasets
eso: deprecation warning for data_retrieval
fix retrieve_data namet
docs: cleanup coordinates import (from astropy import coordinates) and
use 0.4-style coordinate declaration.
Also, added a new index of services by type
NED is also an archive service
docs: rst, coordinate import cleanup
docs: title level consistency
docs: remove some trailing spaces and reorg ogle's title styles
docs: headings in __init__ files are apparently necessary in *some* but
not all cases
docs for classes - NED (I think this may have been the cause of the
inconsistent title error before - classes need docstrings)
docs: conf.py -> mock out bs4
ESO: warnings should only happen if there's actually a failure...
SIMBAD: mask instead of crash for VOTABLE parsing
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.
docs: sections -> chapters in failing regions