Consider `preferred_method="blockwise"` if `by` is sorted

[dcherian]

I'm thinking of groupby("time.year"), which looks like a resample, and also resampling type aggregations,

In general, if by is sorted, then blockwise is potentially the best option.

We could gate this on

1. 1D by
2. ???

I still prefer the idea of using preferred_chunks on xarray's new Grouper objects to rechunk it intentionally but a heuristic might be effective in the intermediate.

method="blockwise" automatically rechunks, so this would be step towards the above.

[dcherian]

With size-1 chunks, e.g.|0|0|0|1|1|1|1, I'm not sure there's any benefit. It would be a regression for when num_blocks_per_groups > split_every

Note that using blockwise is still correct since we rechunk so that all group members are in a single block.