Holds a single, callable, decoder (either a Method+Object pair or just a Method if the method is static)
Static decoder classes should have one (or both) of:
'public static Type decode(String attributeString)'
'public static Type decode(Element element)'
Object decoders should be an object with one (or both) of:
'public Type decode(String attributeString)'
'public Type decode(Element element)'
The decoder will be automatically called whenever a field of 'Type' needs to be decoded.