Cannot infer unit from "micron"

[cmhulbert]

When attempting to export an image as zarr2 with Ome-Zarr-v0.4, I got the following warning:

WARNING: could not infer unit from (micron). Will use it as the unit directly, but may be invalid.

[bogovicj]

Note to self: this has to do with our attempts to convert arbitrary strings to the canonical OME-Zarr unit strings.