simmer 3.4.2

New features:

• Ongoing (unfinished) arrivals are reported with get_mon_arrivals(ongoing = TRUE) (#73).

Minor changes and fixes:

• Simplify Rcpp glue: remove unnecessary as<>() calls (ec4e51a).
• Simplify trajectory's head/tail management (06432a8).
• Now, run(until) runs the simulation exactly until until, instead of until the first event scheduled at a time >= until (e7264f6).
• Fix batch cloning (c20bc1d).
• Coverage improved.