Convert should allow to override UOM

The introduction of the property "Delta UOM", made clear that when an incorrect UOM is set for an analysis result, there is no way (except casting to text and back to number) to change the UOM to a UOM of a different class. We need the convert function to be able to override an existing UOM (class) or ome other way to strip an UOM from a result. E.g. changing a faulty UOM delta T back to T. Refer to 2017SP2 release notes: 121485 A new property, DeltaUOM, has been added to the UOM object to allow applications to determine the appropriate unit for measuring the differences between two values.  For example, temperature units typically have a different unit to represent deltas because of the offset present in the conversion factors.