Implement clone/synchronize activities

[Enchufa2]

Split from #45. clone would cover the duplication behaviour of Arena's separate. Use case: parallel processing of N clones (the original + N-1 copies) and synchronization:

t <- create_trajectory() %>%
  clone(N,
        create_trajectory("original") %>%
          ...,
        create_trajectory("clone 1") %>%
          ...,
        ...,
        create_trajectory("clone N - 1") %>%
          ...) %>%
  syncronize(wait=TRUE)

syncronize would simply remove clones and let the original pass when all clones have been removed.