Add globals before a simulation starts

[Enchufa2]

Method add_global(key, value). Example:

simmer() %>%
  ...
  add_global("key1", value1) %>%
  add_global("key2", value2) %>%
  ...

[Enchufa2]

Functionality:

1. One global at a time (just like regular globals).
2. A value can be a schedule.
3. Defining the same key more than once should raise a warning and do nothing (just like sources and resources).
4. Resetting the simulation environment: reset also these globals, which means:
   • Schedules have an associated manager: no further action needed.
   • Numeric values should be restored, because the simulator clears the attributes (one option would be to define a one-shot manager, so that, again, no further action is needed; as a side effect, 3 is automatically handled due to process name collision).

Requirements:

• R: add_global.simmer, pointing to simmer$add_global.
• Rcpp wrapper: add_global_, or just add_global_manager_.