title: pass a string to function model in fable package
body: <p>I can easily do</p>
  <pre><code>require(fable)
  require(tsibble)
  require(tsibbledata) 
  tourism %&gt;% model(avg = MEAN(Trips))
  </code></pre>
  <p>and it clearly works.</p>
  <p>Instead I would like to achieve something like:</p>
  <pre><code>require(rlang)
  tourism %&gt;% model(parse_expr('avg = MEAN(Trips)'))
  </code></pre>
  <p>but I get:</p>
  <pre><code>Error: Model definition(s) incorrectly created: parse_expr(&quot;MEAN(Trips)&quot;)
  Check that specified model(s) are model definitions.
  </code></pre>
  <p>Thanks for any help</p>