Enforce list of allowed operators?

Right now, there is a list of operators in StockFlow.jl which are supposed to be all those allowed in a stock flow definition, but it isn't currently enforced. EG, the following is valid, even though foobar isn't in the allowed list

k = @stock_and_flow begin
:stocks
S
:dynamic_variables
k = foobar(S)
end

We may be able to add constants by treating them like nullary operators, too, but that's a whole other can of worms.