title: get count() in a groupBy with two facts
body: <p>how can i have the result of a count in a groupBy with two facts.</p>
  <p>Context :</p>
  <pre><code>    private Constraint exampleConstraint(ConstraintFactory constraintFactory) {
      return constraintFactory
              .from(Journey.class)
              .filter((Journey1) -&gt; journey1.isValid())
              .join(Journey.class, Joiners.filtering((Journey1, Journey2) -&gt; (
                      Journey2.getDayJava().equals(Journey1.getDayJava()) &amp;&amp; Journey2.isValid()
                      )))
              .groupBy((Journey1, Journey2) -&gt;  Journey2,
                      (Journey1,Journey2) -&gt; ConstraintCollectors.count())
              .penalizeConfigurable(MY_CONSTRAINT, (Journey2, resultOfCount) -&gt; Journey2.getDelta() - resultOfCount);
  }
  </code></pre>
  <p>Sample of my problem:</p>
  <pre><code> .groupBy((journey1, journey2) -&gt;  journey2,
                      (journey1,journey2) -&gt; ConstraintCollectors.count())
  </code></pre>
  <p>The second group return me a  <em><strong>org.optaplanner.core.api.score.stream.uni.UniConstraintCollector</strong></em> but i need an <em><strong>int</strong></em> or a <em><strong>Long</strong></em>.</p>
  <p>Thanx</p>