Fix symbol indexing by checking that observed !== nothing

[chriselrod]

Fixes #240

[chriselrod]

Sorry about the big diff. I have my editor auto-format files on save.
The change was just adding the A.observed !== nothing checks before dispatching to a method that calls it:

Base.@propagate_inbounds function Base.getindex(A::AbstractDiffEqArray{T, N},
                                                sym) where {T, N}
    if issymbollike(sym) && !isnothing(A.sc)
        if is_indep_sym(A.sc, sym)
            return A.t
        elseif is_state_sym(A.sc, sym)
            return getindex.(A.u, state_sym_to_index(A.sc, sym))
        elseif is_param_sym(A.sc, sym)
            return A.p[param_sym_to_index(A.sc, sym)]
        elseif A.observed !== nothing
            return observed(A, sym, :)
        end
    elseif all(issymbollike, sym) && !isnothing(A.sc)
        if all(Base.Fix1(is_param_sym, A.sc), sym)
            return getindex.((A,), sym)
        else
            return [getindex.((A,), sym, i) for i in eachindex(A.t)]
        end
    end
    return getindex.(A.u, sym)
end
Base.@propagate_inbounds function Base.getindex(A::AbstractDiffEqArray{T, N}, sym,
                                                args...) where {T, N}
    Main._a[] = (A, sym, args)
    if issymbollike(sym) && !isnothing(A.sc)
        if is_indep_sym(A.sc, sym)
            return A.t[args...]
        elseif is_state_sym(A.sc, sym)
            return A[sym][args...]
        elseif A.observed !== nothing
            return observed(A, sym, args...)
        end
    elseif all(issymbollike, sym) && !isnothing(A.sc)
        return reduce(vcat, map(s -> A[s, args...]', sym))
    end
    return getindex.(A.u, sym)
end

I could instead place such a check inside the observed function, in case people are meant to overload RecursiveArrayTools.observed.