BoundingBox should store channel axis information

[markbader]

Detailed Description

• Affected library: webknossos
• At least since the introduction of Zarr as supported format for webknossos the axes order is important metadata. While additional axes as t for time dimension are supported by NDBoundingBoxes now, the previous concept of the channel dimension is still excluded from the BoundingBox. To unify the code and remove magic channel artifacts, it would be great to include the channel axis in the BoundingBox concept.