Hi,

I tried to use the @ode_def macro, but somehow got stuck in an UndefRefError:

```
julia> sir_ode = @ode_def SIRModel begin
dS = -β*S*I
dI = β*S*I-γ*I
dR = γ*I
end β γ
┌ Warning: Symbolic calculations could not initiate. Likely there's a function which is not differentiable by SymEngine.
└ @ ParameterizedFunctions ~/.julia/packages/ParameterizedFunctions/WcTrd/src/ode_def_opts.jl:244
ERROR: LoadError: UndefRefError: access to undefined reference
Stacktrace:
[1] getindex at ./array.jl:745 [inlined]
[2] getindex at ./multidimensional.jl:486 [inlined]
[3] _broadcast_getindex at ./broadcast.jl:596 [inlined]
[4] _getindex at ./broadcast.jl:627 [inlined]
[5] _broadcast_getindex at ./broadcast.jl:612 [inlined]
[6] getindex at ./broadcast.jl:563 [inlined]
[7] copyto_nonleaf!(::Array{Expr,2}, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{2},Tuple{Base.OneTo{Int64},Base.OneTo{Int64}},typeof(convert),Tuple{Base.RefValue{Type{Expr}},Base.Broadcast.Extruded{Array{SymEngine.Basic,2},Tuple{Bool,Bool},Tuple{Int64,Int64}}}}, ::CartesianIndices{2,Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}}, ::CartesianIndex{2}, ::Int64) at ./broadcast.jl:1003
[8] copy at ./broadcast.jl:857 [inlined]
[9] materialize at ./broadcast.jl:819 [inlined]
[10] #ode_def_opts#1(::Symbol, ::typeof(ode_def_opts), ::Symbol, ::Dict{Symbol,Bool}, ::Expr, ::Symbol, ::Vararg{Symbol,N} where N) at /home/joerg/.julia/packages/ParameterizedFunctions/WcTrd/src/ode_def_opts.jl:295
[11] ode_def_opts(::Symbol, ::Dict{Symbol,Bool}, ::Expr, ::Symbol, ::Vararg{Symbol,N} where N) at /home/joerg/.julia/packages/ParameterizedFunctions/WcTrd/src/ode_def_opts.jl:5
[12] @ode_def(::LineNumberNode, ::Module, ::Any, ::Any, ::Vararg{Any,N} where N) at /home/joerg/.julia/packages/ParameterizedFunctions/WcTrd/src/macros.jl:11
in expression starting at REPL[5]:1
```