Adding support for DASCH

I'm currently working on [DASCH], the project to digitize Harvard's collection of astronomical photographic glass plates. Historically DASCH has had a, frankly, very poor data access system, and one of my goals is to fix that. As part of that, I would like to contribute an astroquery module adding support for the DASCH data services.

[DASCH]: https://dasch.cfa.harvard.edu/

A lot of the DASCH data products are fairly typical: we have a bunch of images, from which sources have been extracted, from which we generate a catalog and timeseries light curves. One thing that might be a bit unusual is that we also have a database of plates. Plates are scanned to produce images, but not all plates are scanned, or some are scanned but can't be solved astrometrically, etc. Would recommended approach basically be to implement everything in one big module, or should things like this be split up?

I see your API specification, template module, etc., and I'd plan to start working from those. Would you like to see a PR with early work, or should I wait until I feel like any work is fairly polished? Any other requests or recommendations?

Start with the API specification and template module mentioned in the issue, then review how existing astroquery modules organize access to images, catalogs, light curves, and plate data. Done means agreeing on the module structure and implementing support for the DASCH data services, but the issue does not identify specific files, tests, or a narrower initial milestone.

Written by the indexing model from the issue text.