Update time_var to incorporate default value

[jfree-man]

Update time_var to incorporate the default value of the parameter when change_points[1] > 0. After this change has been made, update the 'Specifying Time-Varying Parameters' vignette here.

> # defaults
> beta_default = 0.2
> beta_changepoints = c(0, 5, 10)
> beta_values = c(0.7, 0.01, 0.4)
> expr = list(
+   beta ~ time_var(beta_values, beta_changepoints)
+ )
> # this works as expected
> simple_sims(
+     iteration_exprs = expr
+   , time_steps = 10
+   , int_vecs = list(beta_changepoints = beta_changepoints)
+   , mats = list(
+       beta = beta_default
+     , beta_values = beta_values)
+   
+ ) |> filter(matrix == "beta")
   matrix time row col value
1    beta    1   0   0  0.70
2    beta    2   0   0  0.70
3    beta    3   0   0  0.70
4    beta    4   0   0  0.70
5    beta    5   0   0  0.01
6    beta    6   0   0  0.01
7    beta    7   0   0  0.01
8    beta    8   0   0  0.01
9    beta    9   0   0  0.01
10   beta   10   0   0  0.40
> # if change_points[1] != 0, we get nonsensical behaviour
> beta_changepoints = c(2, 5, 10)
> simple_sims(
+     iteration_exprs = expr
+   , time_steps = 10
+   , int_vecs = list(beta_changepoints = beta_changepoints)
+   , mats = list(
+       beta = beta_default
+     , beta_values = beta_values)
+   
+ ) |> filter(matrix == "beta")
   matrix time row col value
1    beta    1   0   0   0.4
2    beta    2   0   0   0.4
3    beta    3   0   0   0.4
4    beta    4   0   0   0.4
5    beta    5   0   0   0.4
6    beta    6   0   0   0.4
7    beta    7   0   0   0.4
8    beta    8   0   0   0.4
9    beta    9   0   0   0.4
10   beta   10   0   0   0.4
> # what we want to happen automatically
> simple_sims(
+     iteration_exprs = expr
+   , time_steps = 10
+   , int_vecs = list(beta_changepoints = c(0,beta_changepoints))
+   , mats = list(
+       beta = beta_default
+     , beta_values = c(beta_default,beta_values))
+   
+ ) |> filter(matrix == "beta")
   matrix time row col value
1    beta    1   0   0  0.20
2    beta    2   0   0  0.70
3    beta    3   0   0  0.70
4    beta    4   0   0  0.70
5    beta    5   0   0  0.01
6    beta    6   0   0  0.01
7    beta    7   0   0  0.01
8    beta    8   0   0  0.01
9    beta    9   0   0  0.01
10   beta   10   0   0  0.40