New getters for running arrivals

[Enchufa2]

get_name() for getting the arrival name and get_attribute(), a more general way of retrieving attributes inside user-supplied functions:

trajectory() %>%
  set_attribute("dummy", 1) %>%
  log_(function() paste0(get_attribute(env, "dummy")))

instead of

trajectory() %>%
  set_attribute("dummy", 1) %>%
  log_(function(attr) paste0(attr["dummy"]))

thus deprecating the use of attr and the like, which are obscure, clunky and non-scalable.

[Enchufa2]

Add also get_prioritization().