Cope with simplest delays

[richfitz]

No integration test yet but see it in action:

library(dust2)
gen <- odin({
  ylag <- delay(y, tau)
  initial(y) <- 0.5
  deriv(y) <- 0.2 * ylag * 1 / (1 + ylag^10) - 0.1 * y
  tau <- parameter(10, constant = TRUE)
})

t <- seq(0, 300, length.out = 301)

sys1 <- dust_system_create(gen)
dust_system_set_state_initial(sys1)
y1 <- dust_system_simulate(sys1, t)
plot(t, y1, ylab = "y", type = "l")

sys2 <- dust_system_create(gen, list(tau = 20))
dust_system_set_state_initial(sys2)
y2 <- dust_system_simulate(sys2, t)
plot(t, y2, ylab = "y", type = "l")

and compare with this example from the odin1 docs

[weshinsley]

parameter(10, constant = TRUE) - works correctly!