rely on numpy's version of nanprod and nansum

[keewis]

At the moment, nanprod and nansum will replace any nan values with 1 for nanprod or 0 for nansum. For pint, inserting dimensionless values into quantities is explicitly not allowed, so our version of nanprod cannot be used on pint quantities (0 is a exception so nansum does work).

I'm proposing to rely on numpy's version for that, which would allow pint to customize the behavior.

[dcherian]

Hmm... This will require bumping minimum pint version to avoid breakage: >

???
E   TypeError: no implementation found for 'numpy.nanprod' on types that implement __array_function__: [<class 'pint.quantity.build_quantity_class.<locals>.Quantity'>]

EDIT: I guess this didn't work earlier so we just need to xfail on old pint.

[dcherian]

Thanks @keewis.

It looked like we could remove _replace_nan so I pushed a commit to do that.