Get seized amount from resources

If an arrival is dropped from a resource (see #177) and several resources were seized, this may be needed in order to know which one was responsible.

Also, this should be always possible:

```r
trajectory() %>%
  release("res1", function() get_seized(env, "res1")) %>%
  release("res2", function() get_seized(env, "res2")) %>%
  ...
```