Move fields that are only sometimes needed to the cache?

Okay, how about this?
We reduce fields to the minimum, and everything that is only needed when some method is used goes to the cache. Then we define

@inline function color(system)
    @assert haskey(system.cache, :color) "`color` of a `$(nameof(typeof(system)))` accessed, but there is no color method defined for this system"
    return system.cache.color
end

We could do the same for example with buffer, which is only needed by open boundaries and set to nothing in most simulations. I think this could make the system structs more readable.
What do you think, @LasNikas?

Originally posted by @efaulhaber in https://github.com/trixi-framework/TrixiParticles.jl/pull/539#discussion_r1856702070

Start by reviewing the system structs and existing cache usage, then trace where color and buffer are consumed, especially by open boundaries. Resolve the proposed minimum-field and cache design with maintainers before changing anything; done means the affected structs and cache behavior are agreed and covered by the relevant tests.