update JumpSystem for auto-alg support

Author: isaacsas

src/systems/jumps/jumpsystem.jl

@@ -426,7 +426,8 @@ jprob = JumpProblem(complete(js), dprob, Direct())
 sol = solve(jprob, SSAStepper())
 ```
 """
-function JumpProcesses.JumpProblem(js::JumpSystem, prob, aggregator; callback = nothing,
+function JumpProcesses.JumpProblem(js::JumpSystem, prob,
+        aggregator = JumpProcesses.NullAggregator(); callback = nothing,
         eval_expression = false, eval_module = @__MODULE__, kwargs...)
     if !iscomplete(js)
         error("A completed `JumpSystem` is required. Call `complete` or `structural_simplify` on the system before creating a `JumpProblem`")

test/jumpsystem.jl

@@ -69,16 +69,21 @@ parammap = [β => 0.1 / 1000, γ => 0.01]
 dprob = DiscreteProblem(js2, u₀map, tspan, parammap)
 jprob = JumpProblem(js2, dprob, Direct(), save_positions = (false, false), rng = rng)
 Nsims = 30000
-function getmean(jprob, Nsims)
+function getmean(jprob, Nsims; use_stepper = true)
     m = 0.0
     for i in 1:Nsims
-        sol = solve(jprob, SSAStepper())
+        sol = use_stepper ? solve(jprob, SSAStepper()) : solve(jprob)
         m += sol[end, end]
     end
     m / Nsims
 end
 m = getmean(jprob, Nsims)
 
+# test auto-alg selection works
+jprobb = JumpProblem(js2, dprob; save_positions = (false, false), rng)
+mb = getmean(jprobb, Nsims; use_stepper = false)
+@test abs(m - mb) / m < 0.01
+
 @variables S2(t)
 obs = [S2 ~ 2 * S]
 @named js2b = JumpSystem([j₁, j₃], t, [S, I, R], [β, γ], observed = obs)
@@ -89,7 +94,6 @@ sol = solve(jprob, SSAStepper(), saveat = tspan[2] / 10)
 @test all(2 .* sol[S] .== sol[S2])
 
 # test save_positions is working
-
 jprob = JumpProblem(js2, dprob, Direct(), save_positions = (false, false), rng = rng)
 sol = solve(jprob, SSAStepper(), saveat = 1.0)
 @test all((sol.t) .== collect(0.0:tspan[2]))