Also used to add measurement categories and measurement synonyms. Function returns an empty string upon success.
Function returns the error message as a string when errors occur.
When vName is blank, vCode is used as the Name. Function returns an empty string upon success. It returns the error message as a string when errors occur within the function. If there is already a unit definition with the same Code, that previous definition will be overwritten by the new one. Notice that the Code and Category are the same.