use engine flox for ordered groups

[mathause]

• Closes #233
• Closes #262
• Closes https://github.com/pydata/xarray/issues/7730

Set engine=None and select engine="flox" for ordered groups. Some comments

• this is certainly a very rough WIP
• I think there could be other logic to determine the optimal engine
• I only determine the engine in groupby_reduce however it is already accessed in xarray_reduce - this is not optimal (see below)
• the testing approach is brute force at the moment - I think we'd need to test if the correct engine is choosen (and not just run all the tests)
• so the engine detection should be done in a separate function and then we can test this function
• I tested it and this would solve https://github.com/pydata/xarray/issues/7730 (at least for ordered groupings)

https://github.com/xarray-contrib/flox/blob/5ea713efb5019ff9627893060a6c707a1d14170a/flox/xarray.py#L367

[mathause]

Thanks for the feedback. I have to let this rest until I find some time again - so feel free to take this over if anyone is interested.

[mathause]

I hardly qualify to be on here 😅 thanks for finishing up - this will be a huge win!