Relax Design Implementation to allow strings?

[behinger]

right now our designs work something like this:

@with_kw struct MultiSubjectDesign <: AbstractDesign
    n_subjects::Int
    n_items::Int
    subjects_between::Dict{Symbol,Vector} = Dict()
    items_between::Dict{Symbol,Vector} = Dict()
    both_within::Dict{Symbol,Vector} = Dict()
    event_order_function = x -> x # can be used to sort, or x->shuffle(rng,x)
end

With Dicts of Symbol/Vector.

Is there a reason we only allow Symbols? Or could we relax this to Symbol/String?

I'm asking because Symbol is not a native Python field, so if calling UnfoldSim from python, that is a bit tricky.