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.
Probably should call this `get_image` or `get_image_async`