v0.3.1

Two fixes where the code disagreed with the spec, found in cellmap-flow's migration, and two small additions. No voxel moves in any read.

• source_voxel_size reports the stored level's voxel size under on_mismatch="relabel", as the spec says; it reported the relabelled size.
• box of an array opened with a resample target is the extent of the target voxels shape counts, [corner, corner + shape * voxel_size), as Grid.from_meta(meta, voxel_size=t).box already was. It was the stored data's extent, so read(box) could return fewer voxels than shape: 7 voxels of 8 nm read at 24 nm gave a box ending at 56 nm, now 72.
• The unit table knows kilometres (kilometer, kilometre, km, 1e3 m): they convert, abbreviate to km, and ome_multiscales writes them as OME's kilometer.
• README: the placement rule gains the corner formula, a worked example, why the centre is the model, and how the read rule follows from it.