Support rechunking to seasonal frequency with SeasonalResampler

[dcherian]

Is your feature request related to a problem?

This doesn't work today

from xarray.groupers import SeasonResampler

ds.chunk(time=SeasonResampler(["DJF", "MAMJ", "JAS", "ON"]))

with an obscure error because TimeResampler is special-cased:

xarray/xarray/core/dataset.py

Lines 2515 to 2522 in 8796d55

	chunks_mapping_ints: Mapping[Any, T_ChunkDim] = {
	name: (
	_resolve_frequency(name, chunks)
	if isinstance(chunks, TimeResampler)
	else chunks
	)
	for name, chunks in chunks_mapping.items()
	}

That is a good thing because SeasonResampler sets drop_incomplete=True by default.

We should

1. Raise a better error
2. Probably add another method (memo-ized) to Resampler objects that takes an input tuple of chunks and returns a tuple of chunks, so that SeasonResampler will ignore the drop_incomplete kwarg, and not silently drop data.