title: PyMC3/Arviz: CDF value from trace
body: <p>I have a sample from PyMC3 and I'm trying to get a cumulative probability from it, e.g. P(X &lt; 0). I currently use this:</p>
  <pre><code>trace = pymc3.sample(return_inferencedata=True)
  prob_x_lt_zero = (trace.posterior.X &lt; 0).sum() / trace.posterior.X.size
  </code></pre>
  <p><strong>Is there a better way to do this, either with some helper function from Arviz or XArray?</strong></p>
  <p>I haven't found any <code>.cdf()</code> method or anything similar.
  It's weird that such basic functions are missing, but more advanced ones are there, such as <code>trace.posterior.X.quantile()</code>.</p>