Delegate _reduce to underlying ExtensionArrays

[coroa]

• [ ] Closes # (insert issue number)
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As an alternative to #172 , we are here delegating the _reduce method through to underlying ExtensionArrays like IntegerArray.

I guess there are more functions where such behaviour would be desirable.

[andrewgsavage]

That's a nice approach, will look at this more after pint-pandas is working with pint properly