refactor global reductions to work with `Field`

[jcanton]

the reductions may be better if receiving Fields as arguments instead of NDArray, such that they get domain info that can be used for masks and sanity checks

[jcanton]

we cannot return scalars with field operators, so the factories need to be NumpyFieldProviders, which in turn means that the inputs to these functions can only be ndarrays.

[msimberg]

Just a note: we can always take Fields and access their ndarrays. Using Fields does not imply using field operators. May need some refactoring to allow this in the factories, but this is not fundamentally impossible.

[jcanton]

I agree but thought it was too much refactoring work to change the factories? obviously feel free to reopen this and open one for the factories ;-)

[msimberg]

For now, I have no interest in or urgent need to refactor this so we can keep this closed. Just wanted to point it out so we don't come back to this in the future wondering what the real reason was for not doing this refactoring.

[jcanton]

I'll leave it open then, lest we forget about this