add skip_volume_data argument to Annotation.download() (#806)

* add skip_volume_data argument to Annotation.download()

* update changelog and docs

Author: jstriebel

webknossos/Changelog.md

@@ -15,6 +15,7 @@ For upgrade instructions, please check the respective *Breaking Changes* section
 ### Breaking Changes
 
 ### Added
+- `Annotation.download()` now accepts the keyword-only argument `skip_volume_data`, which can be set to `True` to omit downloading volume data.  [#806](https://github.com/scalableminds/webknossos-libs/pull/806)
 
 ### Changed