Support Field(loc, op) for specifying AbstractOperation location

[glwagner]

This would provide an alternative to @at loc abstract_op for specifying the final location of an AbstractOperation. We can also support Field(loc, r::Reduction).

cc @navidcy

[glwagner]

#2246 renders this less useful but I think it's still worthwhile.

Related is supporting something like

uu = @at (Center, Center, Center) Average(u * u, dims=1)

for specifying the location of a reduction. I think this will work if the reductions support Reduction(loc, field).

[glwagner]

I'm closing this issue because I'm judging that it's not of current, timely relevance to Oceananigans development. If you would like to make it a higher priority or if you think the issue was closed in error please feel free to re-open.