title: ahull_track issue: 'could not find function "psp"'
body: <p>I've been working with the R library <code>alphahull</code> for a few days now.
  As I'm trying to get the results as <code>sf</code> polygons, I'm following the recommandation <a href="https://stackoverflow.com/questions/53472247/r-alphahull-post-processing-avoid-two-hulls">here</a> to use <code>ahull_track</code>, as that returns <code>geom_path</code> objects.</p>
  <p>However, when I try it, I run into this issue:</p>
  <pre class="lang-r prettyprint-override"><code>Error in psp(dat[-np, 1], dat[-np, 2], dat[-1, 1], dat[-1, 2], window = owin(range(dat[,  : 
    could not find function &quot;psp&quot;
  </code></pre>
  <p>I did look at the code of function itself on <a href="https://github.com/cran/alphahull/blob/master/R/ahull_track.R" rel="nofollow noreferrer">Github</a>, and indeed I see it calls <code>psp</code> on line 8, but without any reference as to which library it shouls be loaded from.</p>
  <p>Anyone knows what I missed?</p>