Enforce Member Behavior in Metaclass

I noticed I had made three logical errors when trying Atom (I should have remembered the second from Traits).

- Not instantiating a Member

```python
class Test(Atom):
    b = Bool
```

- Using sub-Atoms without them being Instance() or Type()

```python
class Sub(Atom):
    pass

class Main(Atom):
    sub = Sub()
```

- Not instantiating sub-Atoms (needed to be handled separately).

```python
class Sub(Atom):
    pass

class Main(Atom):
    sub = Sub
```