DiscreteFunction creation fix

[isaacsas]

DiffEqBiological was using the exact parametric constructor form, which has now changed due to the addition of DEStats. This switches to using the more general form with kwargs.

[isaacsas]

Except now syms have disappeared from legends again...

[ChrisRackauckas]

Interesting, wouldn't this give true? https://github.com/JuliaDiffEq/DiffEqBase.jl/blob/master/src/diffeqfunction.jl#L494

[isaacsas]

sir_model = @reaction_network SIR begin
           c1, s + i --> 2i
           c2, i --> r
       end c1 c2
df = DiscreteFunction(DiffEqBase.DISCRETE_INPLACE_DEFAULT,syms=sir_model.syms)

It's the DISCRETE_INPLACE_DEFAULT that messes it up. Using sir_model.f instead causes everything to work ok. Probably why I used the explicit parametric constructor before...