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? 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. Also tests and documentation.