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**: <del>most</del> <ins>all</ins> optional parameters are now supported.
**UPDATE 2**: non-remote tests have been added