into: test foo <- CSV

so if i understand this correctly, if i were to do

```python
h5 = HDF5('file.h5', '/key')
csv = CSV('file.csv')
result = into(h5, csv)
```

then `result` will be an `HDF5` with csv appended to it? this seems a bit surprising to me from a user's perspective, because it kind changes the meaning of `into` to mean "put this thing into that thing" rather than "make the type of this thing the same type as that other thing"