title: ObjectBox, sum a property by grouping another property
body: <p>Imagine I have this table:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>tagId</th>
  <th>amount</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>1</td>
  <td>100</td>
  </tr>
  <tr>
  <td>1</td>
  <td>150</td>
  </tr>
  <tr>
  <td>2</td>
  <td>200</td>
  </tr>
  <tr>
  <td>2</td>
  <td>250</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>And i need to sum the <code>amount</code> by grouping the <code>tagId</code>.</p>
  <p>In SQL we can do this by:
  <code>SELECT tagId, SUM(amount) FROM orders GROUP BY tagId;</code></p>
  <p>Query result:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>tagId</th>
  <th>SUM(amount)</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>1</td>
  <td>250</td>
  </tr>
  <tr>
  <td>2</td>
  <td>450</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>How can I get such results in ObjectBox ?</p>