title: Simmer Get_attribute | there is no arrival running error
body: <p>first of all, this <a href="https://cran.r-project.org/web/packages/simmer/vignettes/simmer-03-trajectories.html" rel="nofollow noreferrer">simmer_vignette</a> and this link<a href="http://r-simmer.org/reference/get_n_generated.html" rel="nofollow noreferrer">advanced_simmer_usage</a> seem to indicate that the error stems from the fact that "get_name, get_attribute and get_prioritization are meant to be used inside a trajectory; otherwise, there will be no arrival running and these functions will throw an error" 
  A minimal workable example: </p>
  
  <pre><code>  patient_traj &lt;- trajectory(name = "patient_trajectory") %&gt;%
    set_attribute("my_key", 123) %&gt;%
    timeout(5) %&gt;%
    set_attribute("my_key", function() get_attribute(env, "my_key") + 1) %&gt;%
    timeout(5) %&gt;%
    set_attribute("dependent_key", function() ifelse(get_attribute(env, "my_key")&lt;=123, 1, 0)) %&gt;%
    timeout(5) %&gt;%
    set_attribute("independent_key", function() runif(1))
  
  env&lt;- simmer() %&gt;%
    add_generator("patient", patient_traj, at(0), mon = 2)
  env %&gt;% run()
  #&gt; simmer environment: anonymous | now: 15 | next: 
  #&gt; { Generator: patient | monitored: 2 | n_generated: 1 }
  
  get_mon_attributes(env)
  #&gt;   time     name             key       value replication
  #&gt; 1    0 patient0          my_key 123.0000000           1
  #&gt; 2    5 patient0          my_key 124.0000000           1
  #&gt; 3   10 patient0   dependent_key   0.0000000           1
  #&gt; 4   15 patient0 independent_key   0.9234335           1
  </code></pre>
  
  <p>Now this works  as it's supposed to work, the problem starts when I try to call get_attribute() in any other sense. Adding this line after set_attribute() at the very end of the trajectory definition:</p>
  
  <pre><code>log_(get_attribute(env, "independent_key"))
  </code></pre>
  
  <p>throws the abovementioned error.
  What I actually want to do is call the "leave" function and give it as a probability an attribute. I still do this in the trajectory.</p>
  
  <pre><code>leave(prob = get_attribute(env, "independent_key"))
  </code></pre>
  
  <p>Needless to say, this also throws the error "Error in get_attribute_(private$sim_obj, key, global) :  there is no arrival running".<br>
  Does anyone know what might cause this? I feel like the only option is the above explanatio "get_attribute is meant to be used inside a trajectory" - but I feel like I am doing this.  </p>
  
  <p>Thanks already!</p>