Improve callback signature

This would reduce the amount of copying data and, more importantly, allocating memory (which should be avoided).

This doesn't make a lot of difference in the example programs in `tests/`, because *views* can be returned in the callback.
But in the more general case (especially if `block_length=0`), memory has to be reserved and an additional copy has to be made.
PortAudio already reserves memory for the buffers, so this should be available directly to the user.

The downside is, that the user now has to use indexing to fill the output array, e.g.:

    output[:] = ...

If desired, I can also add this change to the examples and the documentation.