Save groupby codes after factorizing, pass to flox (#7206)

* Save groupby codes after factorizing

* Fix tests

* Fix cftime resampling

* Fix resampling codes.

* codes is always a DataArray.

* cleanup

* WIP

* Revert "WIP"

This reverts commit c9e2a62.

* Fix doctests

* Fix cftime resampling

Fix cftime resampling

* Better resampling example code.

* Fix test.

* Fix bad merge

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Try fixing some typing

* Avoid warning

* Fix typing

* Add regression  test for #7601

Closes #7601

* Cleanup + fix typnig

* Typing run

* Fix

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Illviljan <[email protected]>