Added methods to LightweightAssetRepr for getting attribute by label

Given that multiple attributes can have the same label rather than start throwing exceptions I thought it was best to make the basic method (getDisplayAttributeValuesWithLabel) return a list of Strings, have a method which returns the first of that list (getFirstDisplayAttributeValueWithLabel) and then methods to check for existence/number of values (hasDisplayAttributeWithLabel & getNumberOfDisplayAttributesWithLabel)

That way it's up to the callers to decided whether they want to check and raise an issue if there are multiple attributes with the same label.