Rollback activity

An activity to go back a number of steps (allow looping...). Use case:

``` r
t0 <- create_trajectory("trauma patient") %>%
  seize("doctor", 1) %>%
  timeout(function() rnorm(1,15)) %>%
  release("doctor", 1) %>%
  timeout(function() rnorm(1,15)) %>%
  # the patient returns to see the doctor again two more times
  rollback(4, times=2)
```
