Correct spelling of ELEMENT.null_flavor attribute to null_flavour

The attribute for representing the 'flavour of null' concept in the ELEMENT class in the Data Structures Information Model was spelled in the US English way, i.e. null_flavor, based on an early idea of reducing differences with the HL7 models. However, it is clear that in recent years the differences are such that this would be of no use anyway. Further, the published XML-schema for the openEHR reference model has always spelled this attribute in the International English fashion, i.e. 'null_flavour'. The rest of the openEHR specifications use international English as well.