Consider RemoteAnnotation structure

[jstriebel]

Detailed Description

Affected library: webknossos

Atm, annotations always contain local metadata and a local skeleton. The volume annotations might be locally available, but not necessarily, and can be streamed from webknossos (see #794). It's hard to communicate and to grasp what parts are fetched at what point, and what is effected by updating webknossos. Therefore it might be useful to have a proper RemoteAnnotation class, which also fetches the skeleton only when needed. Additionally, it would be great to "pin" an annotation to a specific timepoint, to be able to reproduce any previous results, without the need to "archive" the annotation in wk itself.