code: public static String getDataTypeDef(String datatypeKey)
      {
      	Datatype datatype = getDatatype(datatypeKey);
      	return datatype.getDefinition();
      }
docs: Retrieves the definition of a data type.
  
  @param datatypeKey - ID, name or URI (URN or URL) of a data type
  @return definition of the data type
  
  @throws IllegalArgumentException when datatype not found