WIP: Skyview

**What is supported?** You can query the SkyView service by the required arguments position and survey and it will download the corresponding FITS file to the Astroqurey directory.

**What is missing?** <del>Support for optional parameters. The method `_get_default_form_values` already correctly fetches the default arguments and parses them into a nice dictionary, but there seems to be a problem in the usage; need firther debugging.</del> Also tests and documentation.

**UPDATE**: most optional parameters are now supported, though not all yet.
This is what `_async` processes are for.  I'll need to give you more details.