Add convertible temperature UOM to IoX

[Goose66]

It would be handy for node server to have a UOM that they can report in a fixed temperature unit, say Celcius, but that would get converted for display in the Admin Console to whatever "Preferred Measurement System" selected by the user in the Admin Console System Configuration.

[bpwwer]

It would also be good if node servers had access to the "Preferred Measurement System" from IoX.  That way the node server could send data using the user's preferred UOM.