+ [Attribute] added private constants _VALUE_UNKNOWN and _VALUE_ERROR (might...

+ [Attribute] added private constants _VALUE_UNKNOWN and _VALUE_ERROR (might add more specific error values later on)
+ [Attribute] setValueUnknown()
+ [Attribute] setValueError()
+ [Attribute] isKnown()
+ [Attribute] getSynonymWithName(name)
+ [Interpreter] _getExpectedInAttributes(attributes)
# [Attribute] withSynonym(translation) -> withSynonym(attribute)
# [AttributeList] fromAttributeDescription(discoverer, attributeDescription) -> fromAttributeDescription(discoverer, attributeDescriptions)
# [Interpreter] callInterpreter(inAttributes, outAttributes, callback) now retrieves the synonym expected by its inAttributeDescription
! [Widget] check if updateInterval is not undefined when starting the interval