Ensure correct ordering of acquisition in multiplexing converter

[jluethi]

Spotted by Serguei Ovinnikov:

There appears to be a slight bug in the Ome to Zarr Multiplexing converter v1.3.0. It somehow uses some internal indexing based on sorting that mixes up the order. Example: I have 20 acquisitions with names 0,1,2,3,4,5,6,7,8,9,10,11 etc. When I use the converter the order of cycles is 0,1,10,11,12,13,14,15 etc in my zarr file. If I use the 1.2,1 version its works fine.

Let's fix this by going back to ordering after casting to int (as the spec requires acquisitions to be integers anyway).