Allow string coordinates in rolling window

This PR prevents an error when a cube with a string auxiliary coordinate is reduced with the rolling window method. It implements serialization for string coordinates much like `collapsed()` or `aggregated_by()`.

The serializations are chosen to try and best reflect the operation occurring. The bounds are simple, just the same as for numeric coordinates. For points I chose to serialize the whole window, whereas for numeric coordinates the points are the midpoints of the bounds. I feel serializing the whole string is the better choice but I'm happy to be convinced otherwise.