simmer 3.2.0

Major fix:

• In previous versions, resources were monitored before performing the corresponding seize/release activity, before changing the status of the system. Thus, t=3, queue=2 meant that, until t=3, the queue had 2 customers, and at t=3 the system changed (because of a new arrival or a new departure). The idea was to keep the values and time vectors aligned (see #28). But from this moment on, the resources are monitored _after changing the status of the system. This is more consistent with what a user would expect, and more consistent with the behaviour of other related R functions (e.g., see stepfun, from the stats package). Wrapping up and from now on, t=3, queue=2 means that some event happened at t=3 whose immediate and subsequent result was a queue with 2 customers.

New features:

• Add preemption functionality (#34). Preemption comes into play when a resource is specified as preemptive=TRUE. Arrivals in the server can be preempted on a preempt_order="fifo" or preempt_order="lifo" basis. Each seize has three basic properties:
  • priority: already present in previous versions.
  • preemptible: another seize with a priority value greater than this may preempt the present seize.
  • restart: whether the current task (a timeout activity, for instance) should be restarted if the arrival is preempted.

Minor changes and fixes:

• Remove deprecated functions show_activity and show_trajectory.
• Add every, to and from_to convenience functions (8e524cd).
• Fix colour scale in plot_resource_usage (6b034a7).
• Fix compatibility with the upcoming version of testthat (#41).
• The branch activity now provides attributes to its option function, as the other activities (#42).
• Implement error handling in plot_* functions (#44).