Get pixels size units

Add unit handling to Blitz Gateway methods where appropriate.
In progress - PR for discussion only.
Not ready for merging yet.
I what you're effectively looking for a "units.getEnum()" method? (that would be trivial) For the moment, you can likely get it yourself via `getattr(omero.model.enums, "Units%s" % units.ice_staticId().split("::")[-1])`