Neuroglancer Precomputed voxel_offset is not respected for sharded datasets

[fm3]

When loading Neuroglancer Precomputed datasets, the header can have a voxel_offset, compare https://github.com/google/neuroglancer/blob/master/src/neuroglancer/datasource/precomputed/volume.md#info-json-file-specification

webknossos reads this voxel_offset, and holds it as lazy val voxelOffset: Array[Int], but it is not used when loarding sharded datasets. Should be used when loading data etc.

One approach might also be to use the new transformations feature, compare #6748 (not sure if it is the easiest way, though)

[frcroth]

It is already used in chunkIndexToNDimensionalBoundingBox, but that method is not used in the sharded case