add functionality to convert a single round of data to ome-zarr

[kathleenleeper]

It seems silly that in order to get my single round of data into a format that is compatible with a neuroglancer call, I need to go through the process of running initOmeZarr even though there will never be any additional rounds.

[kathleenleeper]

alternatively, clear documentation on how to do this, since clearly the registration notebooks are doing a display function with this data.