Annotation: support empty volumes & make volume location optional

[jstriebel]

Description:

This PR

• adds support for empty volumes in temporary_volume_layer_copy() (bug reported in annotation.temporary_volume_layer_copy() fails for empty volume annotations #810)
• make volume locations optional (preparation for When downloading with skipVolumeData, keep volume tag webknossos#6566)

Issues:

• fixes annotation.temporary_volume_layer_copy() fails for empty volume annotations #810

Todos:

• Updated Changelog
• Added / Updated Tests

[fm3]

LGTM; did not test myself. I assume the get_remote_dataset is now also possible for annotations saved with skip_volume_data? (Note that currently, those are zipped, despite only being a single nml file. In the future, we might opt to not zip those in the backend. would this still be compatible here?)

What is the benefit of changing the BoundingBox staticmethods to class methods? As far as I see it, cls is not used in the methods?

[jstriebel]

LGTM; did not test myself. I assume the get_remote_dataset is now also possible for annotations saved with skip_volume_data?

I tested this with the nml_with_volumes.zip and .nml, this is downloaded from the actual wk PR, and also included in the tests.

(Note that currently, those are zipped, despite only being a single nml file. In the future, we might opt to not zip those in the backend. would this still be compatible here?)

Both would work now.

What is the benefit of changing the BoundingBox staticmethods to class methods? As far as I see it, cls is not used in the methods?

I should use cls for the constructor everywhere, changing this now. This ensures that when calling the method on subclasses (which we don't have atm), an instance of the subclass is created.