Typing of GroupBy & Co.

[headtr1ck]

This PR adds typing support for groupby, coarsen, rolling, weighted and resample.

There are several open points:

1. Coarsen is missing type annotations for reductions like max, they get added dynamically.
2. The Groupby group-key type is quite wide. Does anyone have any idea on how to type it correctly? For now it is still Any.
3. Several function signatures were inconsistent between the DataArray and Dataset versions (looking at you: map). I took the freedom to align them (required for mypy), hopefully this does not break too much.
4. I was moving the generation functions from DataWithCoords to DataArray and Dataset, which adds some copy-paste of code (I tried to keep it minimal) but was unavoidable for typing support. (Adds the bonus that the corresponding modules are now only imported when required).

[max-sixty]

This looks v impressive!

I am traveling for the weekend so won't be able to look at it fully until next week — if someone else could take a glance, then great, otherwise we can merge given @headtr1ck 's track record and I can review next week.

Or if no one gets to this then I will set a reminder for next week.

[Illviljan]

Nice work, @headtr1ck ! I've done a first pass on this and have some minor thoughts.
I'm hesitant on adding #ignores without a comment though. They are easily accepted as truths and forgotten about and sometimes hides real issues.

[shoyer]

I have a couple minor clean-up suggestions, but generally this looks great to me! I can already see several latent bugs that adding type checking fixed.

Thank you @headtr1ck !

[shoyer]

If we remove *args above, this can just be:

Suggested change

	if not isinstance(self._dim, str):
	raise NotImplementedError(
	"Only str dimensions supported by now. Please raise an issue on Github."
	)
	kwargs[self._dim] = upsampled_index
	kwargs["method"] = method
	return self._obj.reindex(*args, **kwargs)
	kwargs["method"] = method
	return self._obj.reindex({self._dim: upsampled_index}, **kwargs)

Which I believe should pass type checking, even for non-string dimension names.

For methods like reindex with a signature like reindex(indexers=None, method=None, tolerance=None, copy=True, fill_value=<NA>, **indexers_kwargs), we generally try to avoid using **indexers_kwargs internally inside Xarray in favor of indexers, for exactly this sort of reason.

[headtr1ck]

I am now getting rid of the _upsample method all together, I found that way cleaner.
If you are ok with this change, feel free to resolve this conversation, otherwise I can revert this commit.

[Illviljan]

This looks good to me now. :)

[andersy005]

Thank you, @headtr1ck!