Issue #2915054 by hanoii, gg24: Allow the field to be used on aggregated views

  /**

   * {@inheritdoc}

   *

   * Only allow the field to be there, no other aggregation make sense

   */

  public function usesGroupBy() {

    return FALSE;

  public function buildGroupByForm(&$form, FormStateInterface $form_state) {

    parent::buildGroupByForm($form, $form_state);

    $form['group_type']['#options'] = [$form['group_type']['#options']['group']];

  }

  /**