Jump simulations are `Float64` valued

[TorkelE]

When you run jump simulations the type of the values are now Float64 (even if initialised with Int64). These are integer-valued simulations, and should have integer values.

using ModelingToolkit, JumpProcesses

@parameters p d
@variables X(t)
rate1   = p
rate2   = X*d
affect1 = [X ~ X + 1]
affect2 = [X ~ X - 1]
j1 = ConstantRateJump(rate1, affect1)
j2 = ConstantRateJump(rate2, affect2)

# Works.
@mtkbuild js = JumpSystem([j1, j2], t, [X], [p,d])
dprob = DiscreteProblem(js, [X => 15], (0.0, 10.), [p => 2.0, d => 0.5])
jprob = JumpProblem(js, dprob, Direct())
sol = solve(jprob, SSAStepper())
sol[X] # Returns `Vector{Float64}`.