Move sliced_mode to output layer

Sliced mode allows us to break up large dask graphs into more manageable pieces by performing the actual computation on only one period at a time. This approach might be made obsolete in the future if dask gains the ability to split graphs. In any case, this should be part of the output mechanism, but at the moment is handled at the graph construction level.

Let's move sliced mode to the output level by integrating it into a save method in main.