Upload annotation to existing projects (metadata parsers)

[dlangenk]

Currently the metadata parsers like the ifdo parser can only be used to import annotations into a new volume. However, it would be great to import annotations to existing projects as well, especially in connection with a COCO parser to import e.g. machine learning annotations.

[mzur]

It should be possible to implement this rather easily with the existing code. When an import is initiated, a new PendingVolume is created from the existing Volume. The rest of the process can be the same.