Support ND for code paths that include `add_mag_for_existing_files`

[MichaelBuessemeyer]

Detailed Description

• Affected library: e.g. webknossos

The current version of the add_mag_for_existing_files method of the layer class does not seem to support nd data. To increase the nd support this should be implemented.

This was noticed in comment

Check list

Paths that include

• layer.add_symlink_mag
• layer.add_fs_copy_mag
• layer.add_layer_for_existing_files