Support axis units

## Current behavior

Image metadata is always read in regardless of axis `unit` metadata. For instance, a 3D image with spacing of 1.0 millimeters along each axis and another 3D image with spacing of 1.0 micrometers along each axis are both read in with spacing "[1.0,1.0,1.0]".

## Requested behavior

My understanding is that ITK image metadata is "assumed" to have units in millimeters for historical reasons. We should parse the units attached to each physical axis and convert the resulting ITK image to have spacing in "millimeters" as standard practice. For instance, the image described above with "micrometer" spacing should be read in with spacing "[0.001, 0.001, 0.001]".

## Additional Notes

The [ngff-zarr](https://github.com/thewtex/ngff-zarr) CLI tool accepts unit inputs for OME-Zarr generation and can be used for testing.