Fix broadcasting for operations that are not part of AbstractOperations.operations

[simone-silvestri]

For operations that are not registered as abstract operations, we do not want to spawn an abstract operation but just pass through the broadcast pipeline with a simple broadcast.
The example is:

using Oceananigans
grid = RectilinearGrid(size = (4, 4), topology = (Bounded, Bounded, Flat), x = (0, 1), y = (0, 1), halo = (1, 1))
field = CenterField(grid)
set!(field, 1.0)
Oceananigans.fill_halo_regions!(field)
field .= min.(field, 0.5) 

The last broadcast does not work in main but works when we bypass the abstract operations pipeline.
However, I have also added min as a @multiary operation (together with max) so that it can operate correctly on multiple fields.

[glwagner]

Doesn't this produce incorrect results?

[glwagner]

I don't think its a good idea to silently allow incorrect operations

[simone-silvestri]

what if we add a warning which explains how to deal with custom functions?

if !(Symbol(bc.f) ∈ operators)
   @error "$(Symbol(bc.f)) is not registered as an AbstractOperation, see @unary, @binary and @multiary [ref...] to use $(Symbol(bc.f)) in broadcasting operations"
end

[glwagner]

what if we add a warning which explains how to deal with custom functions?

if !(Symbol(bc.f) ∈ operators)
   @error "$(Symbol(bc.f)) is not registered as an AbstractOperation, see @unary, @binary and @multiary [ref...] to use $(Symbol(bc.f)) in broadcasting operations"
end

I think that would be nice! We can also check whether we need interpolation. If we don't need interpolation, we can let it pass through (what I assumed would happen before I read the code)