title: Predict_proba for IsolationForest
body: <p><a href="https://scikit-learn.org/stable/modules/generated/sklearn.ensemble.IsolationForest.html" rel="nofollow noreferrer">I try to use an isolation Forest</a> for an outlier detection (fraud detection). If I run the code below (with train and test set):</p>
  
  <pre><code>from sklearn.ensemble import IsolationForest
  iso = IsolationForest(random_state=0).fit(X_train)
  isopred = iso.predict(X_test)
  </code></pre>
  
  <p>I get an array with: <code>array([1, 1, -1, ..., 1, 1, 1])</code>
  which contains 1 or -1. How can I use <code>predict_proba</code> which I can use for <a href="https://scikit-learn.org/stable/modules/generated/sklearn.tree.DecisionTreeClassifier.html" rel="nofollow noreferrer">DecisionTrees</a>. Is there a function available for IsolationTree in the docs it is not mentioned?</p>
  
  <p>When I run: <code>iso.predict_proba(X_test)</code> I get this error:</p>
  
  <blockquote>
    <p>AttributeError: 'IsolationForest' object has no attribute 'predict_proba'</p>
  </blockquote>
  
  <p>I am searching for an array, which gives me the probability which class (outlier or not) the predction belongs to.</p>
  
  <p>My <code>X_test</code> looks like:</p>
  
  <pre><code>A  B  C
  11 1  0
  11 3  0
  11 0  1
  </code></pre>
  
  <p>and <code>y_test.values.ravel()</code>:
  <code>array([0,0,1])</code></p>