set `autofields=True` by default when a class has no constructor

[smarie]

#41 made it clear that a better default value for autofields could be provided when there are no other indications of fields anywhere (no explicit constructor, etc.)

[smarie]

However this kind of "intelligence" may be overkill. Why not instead have an @autoclass decorator in pyfields, using the internal autoclass API ? that would be easier and simpler to understand