[WIP] Test into

Changed one function around. Wasnt working before, fixed it, added chunksize argument. Not sure if we want this to be a pattern, where we add chunksize arguments for functions which use chunks.

Also the code looks ugly and can probably be done better. 
it would be fairly straightforward to write a test that covers this, no?

```python
class A(object): pass
class B(object): pass

def test_into_fails():
    with pytest.raises(NotImplementedError):
        into(A(), B())
```