variable rate jump support

[isaacsas]

Needs SciML/ModelingToolkit.jl#2946 and an MTK release before merging.

Adds a JumpInputs class to auto-select the problem type (i.e. ODEProblem for systems that would have VariableRateJumps and otherwise DiscreteProblem). This now works:

using Catalyst, OrdinaryDiffEq, JumpProcesses, Plots
rn = @reaction_network begin
    k*(1 + sin(t)), 0 --> A
end
jinput = JumpInputs(rn, [:A => 0], (0.0, 10.0), [:k => .5])
@assert jinput.prob isa ODEProblem
jprob = JumpProblem(jinput)
sol = solve(jprob, Tsit5())
plot(sol, idxs = :A)

rn = @reaction_network begin
    k, 0 --> A
end
jinput = JumpInputs(rn, [:A => 0], (0.0, 10.0), [:k => .5])
@assert jinput.prob isa DiscreteProblem
jprob = JumpProblem(jinput)
sol = solve(jprob)
plot(sol, idxs = :A)

Note that plot(sol) does not yet work in the variable rate case, but indexing and plotting of subsets of variables via idxs seems to work ok.