Add other versions of `adder` component

[corakingdon]

We may want to add versions of adder that allow for missing values, or that allow first and last bounds.

examples (need better names):

@defcomp adder2 begin
    add    = Parameter(index=[time])
    input  = Parameter(index=[time])
    output = Variable(index=[time])

    function run_timestep(p, v, d, t)
        v.output[t] = Mimi.@allow_missing(p.input[t]) + p.add[t]
    end
end

or

@defcomp adder3 begin
    add    = Parameter(index=[time])
    input  = Parameter(index=[time])
    output = Variable(index=[time])

    first::Int = Parameter(default=0)
    last::Int  = Parameter(default=typemax(Int))

    function run_timestep(p, v, d, t)
        if first < getindex(t) < last
            v.output[t] = p.input[t] + p.add[t]
        end
    end
end

Note that in this version of adder3, we would need to decide if first and last are index values, in which case we need to add a getindex(t) function, or if they refer to actual year values, in which case we would need to use gettime(t) and this would only work for time indexes that are Ints.