Define the origin w.r.t. the pixel corner or center

The image specification (multiscales) must define the origin, either w.r.t. pixel center or corner.

@lassoan summarized this well in https://github.com/ome/ngff/pull/85#issuecomment-1026030204:

In 2D applications using one of the pixel corners as origin is probably more common, but in 3D imaging voxel coordinate system origin is typically the center. In VTK, ITK, and all applications based on these toolkits origin is in the pixel center. In all 3D image file formats that I know of (nrrd, nifti, metaio, dicom) origin is in the pixel center, too.
NGFF standard must specify this (if it hasn't been specified it already).
If any software that uses different coordinate system convention internally then it can convert to/from the standard coordinate system when reading/writing images.

So I think we should use the pixel center as origin here as well. Anyone wants to make a small PR to add this, @jbms @d-v-b @lassoan @thewtex? Otherwise I can give this a shot once #85 is merged.

Start with the multiscales image specification and read the discussion linked in PR #85, especially the referenced comment. Update the specification so it explicitly states whether the origin is defined at the pixel center or corner, and confirm that the chosen convention is unambiguous.