title: Attempting to save intermediate states when running rmh yields error
body: <p>I am trying to simulate a multitype point process, saving the intermediate states every 1000 steps in <code>rmhcontrol</code>. However, I can't simulate whenever I specify <code>nsave</code>. As an example, whenever I run the code block below, I get the error:</p>
  
  <pre><code>Error in factor(Cmprop, levels = Ctypes) : object 'Cmprop' not found
  </code></pre>
  
  <p>The code is:</p>
  
  <pre><code>library(spatstat)
  library(optimbase)
  num_marks &lt;- length(unique(marks(amacrine)))
  iradii &lt;- .1*ones(nrow=num_marks,ncol=num_marks)
  MSH1 &lt;- MultiStraussHard(iradii=iradii)
  x &lt;- ppm(amacrine, trend =~polynom(x,y,3), interaction=MSH1)
  control &lt;- rmhcontrol(nsave=1e3)
  rmh(x,control=control)
  </code></pre>
  
  <p>Thanks for the help!</p>