Units. DegC, Degc90, DEGC ITS90 and so on

There is little or no control over PARAMETER_UNITS that DACs can choose

Even if we don't have a NVS controlled vocab, these could be chosen once and used by all vendors.

This would only need to be changed if a new sensor measured a new parameter.

It is possible that one parameter could be reported in different units by different sensors
but there could still be a list of unique units that vendors agree to use from the outset.