Throw a warning when an arrival leaves without releasing resources

[Enchufa2]

Leaving without releasing resources is an intended feature, but it may also be a source of problems and unintended behaviours if errors go unnoticed.

[Enchufa2]

Test case:

library(simmer)

t <- create_trajectory() %>%
  seize("dummy", 1)

env <- simmer(verbose = TRUE) %>%
  add_resource("dummy") %>%
  add_generator("arrival", t, at(0)) %>%
  run()

should warn the user with something like "arrival0 is leaving without releasing dummy".