Dynamically disable CuArray scalar operations

[ali-ramadhan]

https://github.com/climate-machine/Oceananigans.jl/blob/fc12f60371eb0df7ba3d088771ddc25c0f258de6/src/Oceananigans.jl#L154

@maleadt pointed out that this line might only execute when the package is precompiled so if we really want to enforce no scalar operations, we should do this in an init.jl file or in the Model constructor or something.

[glwagner]

How about adding a line to the model constructor that executes when arch=GPU(), with a flag that allows the user to permit scalar operations (maybe useful when debugging output / diagnostics)

[ali-ramadhan]

Ah yeah would be good to have the option. It could be a keyword argument of the Model constructor.

[glwagner]

@ali-ramadhan do we still want to do this?

[ali-ramadhan]

Yes, although I think we want to disable scalar operations initially and @vchuravy suggested using @allowscalar on lines where we need scalar operations (currently just one line in each Poisson solver).