Change datamodel.txt to specify pfiNominal/pfiCenter mm, not microns

Datamodel.txt specifies that positions in the focal plane are specified in microns. On the Sept 2021 run (the first time we really designed pfsDesign files) we used mm, and on reflection this makes sense at those are the units used in PFI geometry.

I propose that we make this official, but am of course open to counter arguments.